Is this basic stuff or not? I'm still trying to get into this came data thing myself. I know @DestructionBomb was explaining it to me one, but I probably didn't pay much attention. :-\
As Russian said, it might refer to frame data that is is actually different from the pure numbers shown in the info.
For example, you need to add 2 frames for the animation startup to almost every attack to get the real amount of advantage you need to land it successfully.
Honestly, I have no idea if anything changed about Momiji. I'm playing her the exact same way I did before all the patches. I think they only adjusted some hurtboxes, nothing more.
I just checked the video you were referring to. He just explains that you need add 2 frames to most of the moves (that makes Kasumi's jab 11 frames, correct), because the ingame data is 'incorrect'. Also it takes 1 frame to transition to crouching state.
You can test these "extra" frames when you set the AI reaction to block with fastest SE in training mode.
I'm not 100% sure if this example works with Kasumi too, but if you hit them with her 66KK (no delay for bound) on counterhit, you should have +12 advantage at least.