Grab screenshot keystrokes inadvertently advances to "middle" of Quicktime videos

Hi. I use the keystrokes COMMAND-SHIFT-5 in order to screen grab from videos. Occasionally, it will advance far forward to the middle of the video and leave the original point. For example, I pause the video at 1 minute and use the keys to take a screen shot. Ninety (90) % of the time this works as intended, but occassionally, like after every 9 or 10 captures, it will leave the appointed pause point and advance waaay into the middle of the video. This is very frustrating as it forces me to have to find the original point I wanted to capture. Because I take multiple screen shots I tend to go very quickly, but I've purposely slowed down my keystrokes so I can truly observe to see what makes it advance so many minutes. This last time, I had the SHIFT and COMMAND keys depressed and merely touched the 5 key, and it advanced from the 4 minute point to 54 minute point in the video. I think it's a bug. Any ideas?