Why do testnet blocks have future timestamp?
Testnet is frequently attacked, abused, due to the complete lack of value and proof of work. A piece of code which is intended to allow a difficulty 1 block is none has been found in 20 minutes also enables a person to "warp" forward and mine multiple in a row. If this is exploited over the difficulty adjustment, the entire chain difficulty is reset to difficulty 1. 

The result is that it is trivial to make forks hundreds of thousands of blocks long, with insane timestamps that only vaguely match reality within the consensus rules. They're never going to make a lot of sense as a result. This doesn't happen on mainnet because the difficulty reset code doesn't exist there.