First off, as you have QAD you also have source code to all the reports and inquiries so you could look at the source code for msmsiq.p to see how they calculate it.
Next, are you looking at ATP or cummulative ATP.
With ATP you can only promise something once so when you look at this inquiry it will always show the ATP in its first available week. Example, you have 10 OH plus WO to make 100, you also have a SO for 50, so that week your ATP would be 60. The following week you don't have any sales but the ATP is now 0 because it is shown as available the prior week. Now say we add another SO 20 weeks out in the future for 35. If we rerun ATP again you will now see that the ATP for the first week has gone down to 25 because we do not have any more WO between now and then.
Now the problem you have is that you are looking at the ATP on an individual basis so you probably need to use the cummulative ATP number.
Replied: Apr 18, 2018
Yes, I'm looking at ATP.
Thanks Phill for pointing me to the source and for the elaborate example.