Why would it not be possible for one oil to be less robust than another? AND, YES, some oils are thicker than others.
30wt is a range. If PP & PUP start out at the thin end, they don't have to thin much before they are "out-of-grade" ...within 900 miles in my case. This current fill (a special Valvoline/Cummins oil) thinned less, AND started out more in the middle of the 30wt range. I was wrong above, it went from 11.3 to 10.4...after over 9000 mi.
I've seen others UOA's of QSUD which started out even thicker (11.7?) and thinned to the mid-9's range (dropping ~ 2cst), and Rotella MV start out equally thick but thin to only mid-10 range. In my EB, PUP went from 9.7ish to 8.4 in no time. Also, in my Subaru turbo, Euro Castrol dropped from 12.1ish to 11ish: starting thicker, and thinning less. My first oil change on that car was to PP, and it sounded louder, more-metallic if I recall, though I never had it tested. I know, different motors, different conditions, but I've concluded that some oils are more resistant to thinning than others, AND, if you want to end up "in-grade" maybe it's best to not start out at the thin end of that grade.
