minimum FPS is actually the lowest average in the probing interval. Frames per second are not measured in 1 second intervals, most of the time.
For example:
Game runs at around 30 fps, each frame is delivered every 33ms.
Probing fps with 15 ms intervals would results in:
0ms = 0/3 frames - start point
20ms = 0/3 frames - 0 new frames avg fps = 0
40ms = 1/3 frame - 1 new frame, avg fps = 50 fps
60ms = 1/3 frame - 0 new frames, avg fps = 0
80ms = 2/3 frame - 1 new frame, avg fps = 50 fps
100ms = 3/3 frames - 1 new frame, avg fps = 50 fps
overall:
time 0,1s
frames = 3
avg fps = 30 fps
min fps = 0 fps (at 20ms and 60ms marks)
Ofc noone is doing measurements that often (it was just to show the point that min fps is an average), but 100ms is a default setting in Rivatuner statistics/Afterburner IRC