Official comment
Did you analyse the songs properly and verify the grid was correct? If the song has a live drummer with a varying beat, you need to use DYNAMIC analysis mode. If the song has an electronic percussion with an exact beat, you need to use NORMAL analysis mode. If you use the wrong mode, it may cause the tempo of the track to be incorrect, even if the grid looks accurate.
If you're asking about a track where the song changes tempo, you can manually account for that with this editing trick.