Well it's Linux's whole memory philosophy really. That you ask for data storage that may or may not be memory. This ties in with overcommit, because if you promise more memory than you have then you need a contingency plan. And that means flushing caches, it means swapping data to disk and it means erasing executable code (it is file backed, so it can just be read back in).

This fuzziness of what is and isn't in memory, is why stuff that is rarely needed needs to hit disk meaning a latency spike.