Sorry for the delayed post; been busy and all that stuff, you know!
Although I believe the answer has already been given, I just wanted to toss in something.
Definitely looks (to me) like the Sub Panel is, in fact, connected to ØA and ØC from the Main Distribution (where the Ampere readings were made just outside the CTs).
Bussing in that Main Distribution Panel may have (by the Factory) been setup for the "ØC" leaving the CT Panel Section, to end up in the ØB Position at the Distribution Panel's section (possible, but kind of grasping at straws).
This would be a reality for some gear which may be used on either a 208Y/120V 3Ø 4 Wire Wye, or a 240/120V 3Ø 4 Wire Delta - as the "B and C Phase Position" would be swapped outside of the Metering Section.
Most likely it is simply the position on the Bus, where the Subfeed Circuit Breaker is landing.
If the Gear is using 225 Amp or 400 Amp Frame Sizes, a 2 Pole Breaker might only Terminate to the "AØ" and "CØ" landings on the standoff hardware.
Also, the Subfeed Circuit Breaker may be a 3 Pole, with the Single Phase feeders Terminated to the "Outer Poles" - or across A and C. Some Manufacturers request this type of Termination, when using a 3 Pole Breaker as a 2 pole Unit.
If the Subfeed Breaker is a 100 Amp Frame Size, located on a 42 Space, 250 Amp maximum Bus kit (typical to a common Panelboard), the sequencing of Line Terminations per space locations for 2 Pole Units would be like this:
[*] Ckts. 1-3, or 2-4: ØA-ØB;
[*] Ckts. 5-7, or 6-8: ØC-ØA;
[*] Ckts. 9-11, or 10-12: ØB-ØC;
[*] Ckts. 13-15, or 14-16: ØA-ØB;
[*] Ckts. 17-19, or 18-20: ØC-ØA;
[*] Ckts. 21-23, or 22-24: ØB-ØC;
[*] Ckts. 25-27, or 26-28: ØA-ØB;
[*] Ckts. 29-31, or 30-32: ØC-ØA;
[*] Ckts. 33-35, or 34-36: ØB-ØC;
[*] Ckts. 37-39, or 38-40: ØA-ØB;
[*] Ckts. 41 & 42 would be 1 Pole across ØC only.
Maybe this is of some assistance?