Which is the better estimator for future mining intervals, the 10 minute protocol target or the observed, historical average?
For a good portion of its history the bitcoin network has seen continuous increases in difficulty, which warps the average block time to be below 10 minutes until the next difficulty adjustment. If the reverse were true the block time would be longer in kind. 



This latency serves to protect nodes against isolation attacks where you could otherwise disconnect them from their peers and wait until their relative difficulty dropped to a suitable level for you to mine blocks at. 

The best estimator of the interval is probably to look at the current window and make a guess as to the direction of change, but this is very error prone due to the low sample size of the block interval.