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I check the manual of ps, and it said -e will list all processes, while -u will list processes for a specific user.

So I try to use ps -eu myaccount | wc -l and it outputs 280.

But if I use ps -u myaccount | wc -l and it outputs 12.

Since both are used to output all the processes for the user myaccount, why the result will be different?

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A better question would be "What's the difference between ps -e and ps -eu" and the answer would be "nothing".

The -e option shows all processes, and basically replaces the -u selection.

As pointed out by @fra-san, the man page indicates all process selection options are additive. The -u option selects, not filters, and you can't add any more processes after -e as they are already all there.

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    It is also worth to note that on a multiprocess system, which Unix-like systems are, the number of processes might differ between two invocations of ps. Also the header line will be counted by wc.
    – Vilinkameni
    Oct 22, 2021 at 10:22

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