Why is average time for block 8 minutes, when the target is recomputed to keep it 10 minutes? I found out that the average time per block (based on the last 10 000 blocks) is 464 seconds - something under 8 minutes.

http://blockexplorer.com/q/interval/10000

However, I thought that the target recomputing is done so it is kept at averagely 10 minutes per block. Where does this difference come from?
The network difficulty is recalculated every 2016 blocks.  The difficulty is adjusted based on the duration of the most recent 2016 block interval.  The difficulty can adjust up or down at most by a factor of 4.

Currently, the network is growing rapidly.  The network finds blocks at a faster rate than one per 10 minutes because the current difficulty is based on the hashrate of the previous difficulty interval.