Beat Jump by BPM or Beat Grid?

Hi,

Does anyone know, in Rekordbox 6 (I'm using it with the DDJ-400), whether the 'Beat Jump' function works according to BPM or the Beat Grid?

Because, on tracks where the BPM is variable - but I've spent the time to beat grid it - when I use the beat jump, it doesn't jump to the corresponding beat (on the grid), but some random place close by; then... the strange behavior continues... when paused, if I 'beat jump' forward however many beats, and back, it's not the same place.

Any thoughts or understanding on why it doesn't work as expected.

Thank you in advance.

Matt