Usable space: PW vs Oasis 1 vs ???

hi all,
just a random observation. i looked at my PW3 which has roughly 200 books on it, and it reports that i have 2.7gb of free space.

my recently purchased Oasis, even after updating (not partial update file sitting on the directory structure), and having only 5 books on it, reports having 2.4gb of space available.

is the OS/Firmware on the Oasis that different than the PW? (PW3 in my case)

thanks for any thoughts/experiences. it doesn't impact me because i don't forsee running out of space for a long time and am happy to offload things into the cloud, but i found this discrepancy peculiar. i could have sworn that my PW3 came with over 3gb of usable space before i put books on it.