OK...perhaps someone can explain this query:

I'm checking an upcoming itinerary PSP-IAH-NYC. In ITN, there are three different connections through IAH to EWR or LGA, all from CO 1850 on the PSP-IAH leg - and each of the three shows different availability for the exactly same flight:

1) A6 D5 F1 E0 Y9 H9 K9 B9 V9 Q9 T9
2) A6 D5 F0 E0 Y9 H9 K9 B9 V9 Q9 T0
3) A6 D3 F1 E0 Y9 H9 K9 B9 V9 Q9 T9

How are you supposed to know how many seats exist in a class, if the availability is completely based on what other flight it's paired up with? Why would upgrade availability vary by what other flight it's paired with?

Can anybody explain this one?