Why are the rules different?
There are several rules in Online Survivor that differ from the real Survivor's set of rules. To understand why, you must first understand the entire concept of Online Survivor. Since 2000, Online Survivor has strived to stay true to the original set of rules that Survivor established in its early seasons, then do "twists on the real Survivor's twists" each time a new concept is added to the game. Therefore, Online Survivor has almost never copied a twist from the real Survivor, no matter how simple the twist is. This is why things that seem like "standard Survivor rules" to recent fans of Survivor are not present in Online Survivor. Things like the Rock-Picking Tiebreaker and Hidden Immunity Idols are not part of Survivor's original rules, and are actually twists that were permanently added as late as season 12. Thus, each rule is in place simply because Online Survivor has established a tradition of doing twists on the real Survivor's twists, and then keeping that "twist on the twist" if it is used again in the real Survivor. This is WHY the rules are different, but if you're more interested in HOW Online Survivor's rules came to be, then continue reading. I think you'll find that most of the alternate rules actually have a logical explanation that stay true to Survivor's core, despite how crazy they may seem to some people. The following is a description of each contrasting rule, and how it came to be present in Online Survivor.

Rule 6.2: Tie Vote Origin
Quite simply, this was Survivor's official rule in its first three seasons. Go back and watch season 2 or season 3, and you'll see this rule play out. As it is part of Survivor's original format, Online Survivor has never deviated from it.

The rock-picking tiebreaker is not a tie-breaker at all, but a tie-ignorer. Someone who didn't even receive one vote at the Tribal Council can be randomly eliminated. This makes no logical sense at all, is completely unfair, and destroys the core "tribe has spoken" concept of Survivor.

Rule 6.3: Hidden Immunity Idol Origin
At the time Hidden Immunity Idols were unveiled, there were only two types of Immunity: Idols, which protected a group of people, and Talismen (or Necklaces), which protected an individual. In creating this twist, it was first considered that idols were meant to protect a group of people. In much the same way that the Immunity Idol works in the tribal stage, the same exact rules were applied to the Hidden Immunity Idol. The contestants would be split into two "temporary tribes". One would have immunity and not attend Tribal Council, and the other would be vulnerable and be forced to vote someone out of their "tribe". The only thing left to determine was how to divide these "temporary tribes". The most obvious solution was to divide them based on who voted against the idol holder and who didn't. This is an obvious division between two groups of people, and all this rule does is apply the tribal stage rules to one round of the game.

The real Survivor's hidden immunity idol was put in place to give the minority alliance a chance to flip the game. However, it has failed to do so. Most people know how to get around the idol by splitting the vote, and even if the idol is used successfully, not much is accomplished other than cheaply eliminating someone from the majority alliance. Afterwards, the alliance usually stays intact and continues to dominate anyway. Not to mention, the idol is a very cheap mechanic, in which someone can be eliminated by a very small amount of votes (even 1 is possible), which pretty much destroys the majority-rules concept of Survivor. Even Jeff Probst has stated that the idol doesn't do much anymore, and they're looking into changing its rules. Online Survivor's rules accomplish what the idol was meant to do: It gives the minority a chance to flip the game by causing the majority to turn on each other prematurely, exposing cracks in the alliance. It also does so fairly, in that the person eliminated will receive the most votes at the new Tribal Council, not the least. Cheaply picking off someone with 1 or 2 votes doesn't change anything.

Rule 6.4: Mass Immunity Origin
This one is a unique case. The real Survivor never encountered a situation where there was mass immunity until the finale of season 34. Online Survivor, on the other hand, encountered this much earlier, and created its own rule at the time. If all but one person is immune in the real Survivor, that person is automatically eliminated. In Online Survivor, this occurred about 2 years earlier, was met with criticism, and rules were created to ensure it wouldn't happen. The game of Survivor is about voting, and manipulating people's votes, not about an Easter Egg hunt of who can find the most immunities. This rule gives everyone a fighting chance, and ensures they can only be eliminated through a vote, and not by default.