And a GM Techlink article:
Seems to me that in both cases the cap is configured to allow bypass to occur if a high enough differential is achieved; the cap provides no seal to the top of the filter.
- On the one, the filter top seals against the centre tube, where the bypass is located, and this will open once a sufficient differential occurs.
- On the other, there is no seal between the centre tube and the filter top, it's wide open, because the assembly is sealed on the bottom and the bypass is located on the top of the filter.
So, using the filter without the bypass on the top on a housing that requires it allows for unfiltered oil to circulate constantly. If it was done the other way around, suspect, if you could get it together, that you'd not have an operational bypass.