Hi Toril,
If you look at the voice type of those voices on Genos and Tyros5 that produce an auto 'glissando' when two notes are played one octave apart (actually from a minor7th to an octave apart), I suspect that you will find that they are Super Articulation 2 (S.Art2!).
The PSRs (even the SX models) do not have S.Art2! voices. Although your S975 does have S.Art! voices. these do not have the auto articulations.
Regards
Ian