Converting a 120 fps clip to 24 fps by changing sequence settings

When you cut a clip into a sequence, if the frame rate of the clip doesn't match the rate of the sequence, Premiere Pro will automatically skip or hold on frames where necessary to match the original temporal cadence so the video still appears to play in "real time".

In your example, there are 5x as many fps in your 120 fps clip as in your 24 fps sequence so if you go frame by frame through the sequence you should see only ever fifth clip of your source clip. Premiere is just skipping four frames each time, to keep up the same rough amount of clock time as was happening when the video was recorded.

The inverse also happens. If you cut a 30 fps clip into a 60 fps timeline, Premiere Pro will hold on each source frame for two frames in the sequence.