Reporting the clock frequency of a core requires running a bit of code on it which necessarily means the core is in an active state while you take the reading,

Reporting the clock frequency of a core requires running a bit of code on it which necessarily means the core is in an active state while you take the reading, so you will always read something like "2.5 GHz" even if the core does not execute anywhere near 2.5 billion cycles while you do that. Some tools can report metrics like "average effective clock" which count time spent in inactive power states as zero clock.