Circ, Robo is right. (O.o)
From frame data, Pot's 2K is a Lv2 attack which gives it 12f of hitstun vs standing. [link]
From Pot's frame data, 2D and far S have 12f startup, whereas 2H has 13f. [link]
2K->far S and 2K->2D both combo, but 2K->2H doesn't. Meaning that in GG frame data an attack with 12f of hitstun combos into an attack with 12f of startup, so the last 'startup' frame is really the first active frame.
BUT HOW DOES THIS WORK WITH YOUR VISUAL RESULTS? Well, I'll tell ya, since it works the same way in SG so I can explain it. (^.^)
In lots of games such as 3s/MvC2/SG and now apparently GG, frame 12 is the first active frame but the hit detection is done after the characters are advanced on that frame, so it SHOWS the hit on frame 13 since that's when everyone reacts...but if you held Back to block on frame 13, you would still have been hit since the hit actually came out and made contact on the previous frame. Make some sense?
So if they did the frame data by gauging it visually when it looks like it hits, they would list the attack as having 12f of startup because visually the opponent reacts on 13, but what's really happening inside the engine is that the opponent is getting hit on frame 12 after everyone is advanced, thus the reaction happens visually on the next frame. Frame 12 is the last frame of hitstun (so on 13 they would be in idle stance if the they weren't hit), but it is also the first frame the next attack goes active, though the actual "did anyone hit" calculation is done after all the sprites are advanced for that frame.
[edit] And so, in Guilty Gear XX Accent Core Plus Reload (whew), Pot should be able to do 5P->2S vs air.