OT Am I waiting for data or not?

I suppose this is a webbrowser question but my setup to ask FFox is
still on the backup HDD.

How is it, when watching a video, one that has two progress bars, one
that shows how much you've watched and another right underneath it that
shows how much you've downloaded, how is that even when you've
downloaded 4 minutes more than you've watched, the playback will still
stop and a spinning circle show up for 10 seconds or more??

I thought that meant it was waiting for more video to download, but the
progress bar shows that several minutes already have. ????