> everyone sees the same data in block number X
The same data in a specific block, but not everyone necessarily agrees that the same block is the latest one. That's why you don't have 100% confidence in a single block confirmation.
Exactly, what I mean by "finalized" is that everyone agrees and there's a very low probability that they'll change their minds.
If a block isn't finalized, and you build a transaction based on particular conditions in that block, and the finalized block has different conditions, then your transaction just fails.