I have noticed that then I do a flight availability search I may get different results if I search for a connecting flight than if I search for each leg separately.

For example

If I search msp-dtw with a connection in atl, I may get y=1 on the msp-atl and y=0 on atl-dtw.

Yet if I search just msp-atl I may see y=9 and searching atl-dtw may be y=3

As delta allows sdc for y>0 sometimes I may not see open seats that are there.

What cases this? Any solution other than a two step process?

Thanks