Best answer by Airgetlam
Bonded speakers (the SUB, surrounds) communicate on a low latency network of 5 GHz, so they’re in sync. There may be some aspect of because they’re few in number, it makes a difference, too.
Grouped speakers will communicate on the 2.4Ghz channel. Sonos software will buffer a certain amount, in order to ensure that the stream can be in sync across up to 32 other speakers. The delay is on the order of 70ms, and there’s no way to get rid of it.
Your system is working as properly, just probably not the way you want.