What's your BPM?

As an editor of both video and music you may want those beats to line up time intervals like seconds or half seconds. You pick a "reasonable" music temp of 120 BPM BUT your NTSC video is 29.97 fps, not 30. You'll find that 119.881 in STP will get you closer. At this tempo you'll have a beat every 15 frames. It'll still slip a bit as time goes on but STP tempo doesn't resolve past the third decimal place.

The key to remember is that NTSC video is 29.97fps (drop frame time gives you accurate running time) so you'll want to use fractional tempo for beats to hit on specific time intervals.

Of course if you've ever wondered why some people use Non Drop Frame (30fps instead of 29.97) time code, it can make the math easier.