Linux – How to view total CPU and memory usage of per system user in Linux

You should add a snapshot of tops output to your question so we can see exactly what you see.

The most common discrepancy in processor use indicators is some counting "I/O wait" time as well as "actually CPU busy" time. I/O wait time is when a process would be actively doing something, so could potentially have used all the time-slice the scheduler has given it, but has yielded as it is blocked waiting for I/O operations to complete.

Short running processes have an effect too: if there are many processes starting, doing something small, then closing, they may not be seen at all by top or similar (especially if you don't have top updating often) because they did their thing and stopped between updates. Even though they were not present when top took its samples (so don't appear in the tasks list) they overall counts in the /proc filesystem will still show since the last sample the CPU was busy for X scheduler ticks and idle for Y.