I appear to have a gear inbetween 3rd and 4th on 1997 626 Auto

When travelling on flat road at say 80kph, I have approx 2000rpm on taco

When I then come to a slight hill, and accelerate slightly, transmission feels like it drops back a gear, any engine goes to approx 2500rpm. So far do good.

If I then move gear selector down one gear (or press hold button), the transmission downshifts and engine revs go to approx 3000rpm. I would have though that if already in 3rd, and I move selector to third, no change should be evident.

If on the other hand, I press hold button/down shift manually when doing 80kph on flat, engine revs go straight from 2000 to 3000.

Any thoughts on what is happening?
Thanks
Andrew