Apparently there was a screw-up with the tickets. Some do in fact say:
SECTION - STANDING
While others say:
STANDING GENERAL ADMISSION
I'm guessing they will check tickets and wrist-band accordingly upon entry, but the entrance is the same. I've been to the O2 a lot and Entrance F is the main entrance to the floor, regardless of which section you stand in. Security at the reflection section inside the arena will check writsbands before letting people into that gated-off section.
I'm sure everyone going later in the week would appreciate feedback from those going today as to how it actually works.