There's another reason too; there are strong reasons why the timing of successive code executions should not be linear. Typically the first call to a function will incur the cost of fetching data into a cache. At minimum the basic blocks for the function will be loaded through the l1/l2/l3 pipelines.

2, 3, 4 function calls in a row will benefit from that caching, but also help train the speculative execution engine. At some point that speculation may radically alter performance. Later, with a sufficient number of runs, the timing will also suffer from preemption, context-switches, or other interruptions that can invalidate both the cache and the speculative execution engine.

The idea that there is a fixed performance time for a function is naive on modern hardware.