coil design

Does anyone know why some units have been designed with the 'air on' on the refrigerant in side, the liquid side of the coil? i was taught that the air flow is always meant to be from the warmer side, the suction side of the coil going through to the colder side the liquid in.

Nope, even manufacturers agree with you, see page 4.
http://www.colmaccoil.com/HeatingCoo...NG00013632.pdf

Hi baba jay.
Brian_ UK 's post is totally correct, DX evaps are at max efficiency with air counter flow to refrigerant flow.
Is it a case that someone else had a fan or two that fitted the hole and did not check air flow direction. Because dependent on application parallel air to refrigerant could be correct. can you describe and explain system. More information would be good.

Usually you want to maximize heat transfer (energy efficiency) and for that you need the maximum temperature differential you get from counter flow.

But sometimes you have other objectives like not frosting the coil or to minimize dehumidification so you design for the minimum temperature differential.

Or maybe design for a certain defrost technique then you use down feed instead of up feed.

Different designs for different objectives.