That is exactly how it should work.
You foot applies more tension on the bottom belt which forces the sliding member up.
This changes the relative sizes of the pulleys which changes the speed.
So your foot has to stay on the speed pedal which is why they put cruise control on those models.
There is an adjustment to compensate for belt wear.
Apart from that wear in the belt, pulleys or tensioning arm pivot will cause a lack of sufficient tension in the front drive belt.
