This percentages you're giving are very strange, specially with ANC on!
I'm looking at the percentages since my first full charge many times and, in general, the percentage showing is close with consistent accuracy, always changing in very small increments, but I know isn't perfect still, of course, but is, in my experience, many better than majority of others BT (ANC) headphones with this type of information.
Is strange that b&w advertising 30 hrs with ANC on but they're don't saying nothing about when ANC is off, and after doing my test, I think this is very good marketing for b&w because (many?) people, like me, aren't always activating ANC or transparency.
The H95, for example, is advertising 38 hrs with ANC on and 50 hrs with ANC off (in my test is more close to 60 hrs with no ANC or transparency). So, the difference in the h95 is about 30% between ANC on or off in their documents (even when I testing 60 hrs with ANC off). So, if at the end I getting about 45 hrs, with ANC off, minus 30% is 31,5 hrs for px8, so maybe b&w is giving good estimate.
(I will not testing px8 how many hours with only ANC on with 55% volume because I don't have this necessity, but I'm curious now.)