It's not confusing at all. The of the all corners oriented OLLs there are 7 unique cases (8 including OLL skip). You can have adj in FR, BR, BL and FL meaning there are 4 ways to get OLL 28
Since there are 4 ways to get it, we can infer a 4/216 probability, which reduces to 1/57.
For the two opposite edges case, OLL 57
there are only two ways it can appear, FB and LR, because a y2/U2 will give you the exact same case, unlike the adjacent edges case. 2/216 = 1/108
For the no edges all corners case, OLL 20
there is only one was for this case to appear, as doing a y,y' or y2 doesn't change arrangement of oriented/misoriented stickers. 1/216