Dvd Frame questions?

I notice that when I frame advance certain NTSC dvds that there is a
change in movement between every frame(the newly released NTSC version
of "the Adventures of Robin Hood" behaves this way) I notice on other
film originated NTSC dvds including some of the extras on the "Robin
Hood" dvd every 4th frame is repeated. In other cases when you
transfer the Dvd to VHS video the transferred VHS tape repeats every
4th frame but the original Dvd doesn't; when you use the frame advance
there is a change in movement for every push of the "frame advance" on
the original dvd.

Can somebody kindly inform me as to what is going on here?

Regards,
Peter Mason