Why is segwit support rising so smoothly? I expect to see some noticeable random steps (e. g. when some relatively large pool on miner starts to support segwit). Instead I see almost perfect linear dependency. Why is that? I am afraid that I miss something fundamental about segwit.

I painted in red what (approximately) I expect to see, while blue line is the real one. Graph is taken from https://blockchain.info/charts/bip-9-segwit
because the number of supporting pools is a constant

and the percentage is calculated by last 2016 blocks after the start of signaling