Various research has shown that the variation can be much higher than 15% due to things like this. It's not that rare; I keep bumping into it. Compilers and linkers do a reasonable job but fundamentally modern CPUs are extremely complex beasts.
I found Casey Muratori's series the best explanation of what is going on at the CPU level.
I found Casey Muratori's series the best explanation of what is going on at the CPU level.