The block time isn't exactly 10 minutes. Blocks are mined (roughly) every 10 minutes on average.
A block may be confirmed a few seconds after the previous one. It's also possible that it takes more than an hour.
I want to get a little explanation about this case:
When the user pays the optimal fee so that his transaction will go in the next block, what makes the next block confirmed more than 1 hour?
My assumption: On average each block will be confirmed in 10 minutes, but when the transaction uses the optimal fee to be entered into the next block, then I think the transaction will not take much time to get confirmation. If my statement is wrong then I hope some one can correct it.