Not frame advantage, blockstun.
Here, look at this. This might help you understand. This is for Valkenhayn in BlazBlue Centralfiction.
Let's take the following string as an example>2A-5B-5C-6C
and split it up like this>2A-5B
2A has 11 frames of blockstun and 5B has 10 frames of start-up, so if you cancel it ASAP, there is no gap because 10-11=-1>5B-5C
Again, no gap because 5B has 16f blockstun and 5C has 14f start-up. 14-16=-2.>5C-6C
This time there's a gap because 5C has 18f blockstun, while 6C has 19f start-up. 19-18=1, so there's a 1 frame gap.
I think it might be calculated from the last active frame, though, so that math might be slightly off. I'm not entirely sure on that. You get the point, though, yeah?