In the book, Computer Organization and Design, Fifth Edition, by D. Paterson, it is stated that
Differentiating between system and user CPU time is difficult to do accurately, because it is often hard to assign responsibility for operating system activities to one user program rather than another and because of the functionality differences among operating systems.
From how I understand things, this is due to the fact that different operating systems have different designs. Different OSes may use user programs to handle OS activities like file system management and graphics, and use the kernel itself for other activities. Is my understanding correct?
Additionally, an elaboration on
...and because of the functionality differences among operating systems.
in relation to the difficulty of differentiating between system and user CPU time would be nice.