Computer architecture: why is it difficult to differentiate between the CPU time of the system and that of the user?

In the book, Organization and computer design, fifth edition, by D. Paterson, it is stated that

It is difficult to differentiate between the system and the user's CPU time
accurately, because it is often difficult to assign responsibility for
operating system activities to a user program instead
and due to differences in functionality between operating systems.

How I understand things, this is due to the fact that different operating systems have different designs. Different operating systems can use user programs to manage system activities, such as file system management and graphics, and use the core itself for other activities. Is my understanding correct?

Additionally, an elaboration on

… and due to differences in functionality between operating systems.

in relation to the difficulty of differentiating between the system and the user's CPU time would be good.