I think whats happened here is this: Patlabor was originally an OAV series. It was so successful that a TV series was produced, but existing in a seperate continuity to the original OAVs (thus needing to re-introduce the characters). After the TV series another OAV series was made, this one actually following on from the TV series.
A similar thing occurred with Tenchi Muyo!.
Possibly the "1st Season" and "2nd Season" are the US distros labelling?
There's also a third separate introduction in the manga, which was released concurrently with the original OVA.