No idea, friend :D
Apparently, they wish to make it difficult for users to predict everything. We can surely do it manually. The rating in a play-offs match has to be scaled with the number of maps played in that series, all of it has to be summed up, and divided by the total number of maps. There we have it, play-offs rating.
The only problem was, people didn't know which matches counted as playoffs. Many counted the full tournaments like Blast Finals, IEM Global Challenge, etc, as play-offs. However, that's not true, as has been officially confirmed.