Okay, so I checked if an extra frame is on the rendered video by putting a 3.000 second image in the sequence. Then I try to export the video on 1280x720 resolution (not that resolution matters) at 60 frames per second. Constant frame rate is checked (not that that matters either).
I open the exported video on VideoPad, the length is 3.017. If I export on a smaller frame rate, this extra frame will be bigger. Say that I now try to export at 30 frames per second, the length is 3.067.