Confusion about frame data

So Djeetas 66l is +6 on block
Her fM has it's first active frame on frame 8

So if I do 66l > fM, their should be a 1 frame gap where my opponent can block the fM, even if hit by the 66l right?

But when I set the dummy to block after initial hit and do this, sometimes it the fM, other times it gets hit by it?

What don't I understand??