I'm not entirely sure how Kobo measures and estimates reading time but I'm afraid in my experience I've found it completely inaccurate to the point where I just ignore them
. It's a nice to have feature for just eyeballing whether I can sneak in a chapter for my train ride but doesn't bother me massively if I don't have it.
My old Kindle let me (not sure if this is still a thing) reset and calibrate reading speed for pretty accurate time estimates which was nice but I found Kobo's (Nickel) handling of x turns === x pp. versus whatever Kindle was doing to be something I personally prefer over having more accurate reading time estimates.