https://blog.cotten.io/ethereums-eip-2-4-15-second-block-target-98d4c11017e1

This article explains the difficulty adjustment algorithm employed in Ethereum since the Homestead release on March 14th, 2016. The achievement of an average 15 second block solving time is explored in detail.

Ethereum had a block difficulty targeting problem before the Homestead release: it relied on a median target of 13 seconds but the average was drifting higher and higher.