Twix marketing plays up the joke that the left bar and right bar are vastly different (one advertisement shows them supposedly being made in separate factories) when in reality they're exactly the same. This tweet posits the scenario of a study being done to see if people can guess which bar they are given, even though, again, there's zero difference. Essentially, it's a 50/50 shot. Getting it correct over 300 times is technically plausible because of probability, but practically speaking, it's impossible.
tl;dr the joke is about someone correctly guessing which half of a chocolate bar they're given, when both halves are identical.
tl;dr the joke is about someone correctly guessing which half of a chocolate bar they're given, when both halves are identical.