However, there is something I didn't understand (not sure if it's the same thing pooya87 pointed out in his repy!): when you click on the first bus, which is supposed to represent the next block, you can see a list of transactions that have been included in it!
AFAIK each miner/pool select transactions as they see fit and each miner/pool may be working on a different block with totally different transactions.
Not sure but my guess is that the website is simply adding transactions paying higher fees first!
I think you are right on this one that they are prioritizing or "assuming" that these people/transactions are part of the block since they have paid a higher fee which I think is reasonable.
This is what they said on the About section.
If the transaction has a high enough fee, they will board the first bus and be ready to be included in the next mined block. If there are too many transactions to be included in the next block, and the transaction didn't pay a high enough fee, the person will either wait in line or board a different bus.
The movement speed of a person represents how high of a fee they paid compared to the current median fee.
Everytime the bus is moving you will immediately see the animation of people having faster walking speeds compared to the others and they usually skip some people that are lined up which just indicate that they have paid a higher fee. Of course these transactions might not be really included in the block but I think what they just want to visualize is how the transactions work in this kind of representation.