Then I asked myself, why? why? :-(...
Up to this moment, my best guess is: That simptom is caused by a VBlank interrupt.
Because the main operation performed by the official controller library occurs once per frame via the VBlank interrupt.
And the sceGsSyncV related function, waits for sync with V-Blank.
If a title originally outputs an interlaced mode (ex.:NTSC480i), it sets also interlaced on and field mode (alternated even/odd fields) for SetGsCrt parameters.
For interlaced modes, sceGsSyncV return 0 (Even field) and 1 (Odd field) values.
When forcing outputs to a non-interlaced mode (ex.:HDTV480p), it sets also interlaced off and frame mode for SetGsCrt parameters.
Then, sceGsSyncV will return always 1 (for non-interlaced mode).
If a interlaced game forced to be a non-interlaced one, expect both values, unprectictable things should happen... BSOD and no gamepad response, for instance!