That’s one of the most interesting myth of bitcoin, it’s block time interval although averagely set at a default of 10 minutes. The time varies because of the ability of a miner to get the correct block hash that Should he below the target value. It is this target value that is adjusted every 2016 blocks just to keep the 10 minutes average in check. Before the block time interval sometimes was higher due to maybe lack of hash power or lack of many miners. This days we have quite a number of mining pools with high hashing power so the issue of 2 hours block time interval isn’t that much common again. But still we can’t get a definite time interval because of the target that needs to be meet which depends on the miners finding it.