PWA breaks this probability into Win and Loss Points and uses the ratio of Win Points to the sum of Win and Loss Points. PGP uses the probabilities directly. So, the probabilities for PGP were calculated from a separate set of data. I prefer to see the probability with respect to a game because of its more general applicability to any game in the season. It is bounded by any probability being between 0 and 1.
Baseball is prone to wild swings in the game with individual plays.
PGP reflects the nature of the game. Over a short series of games, it is possible for a mediocre player to shine and achieve a PGP value much higher than expected from his ability. In some ways, it is this variability that is the distinguishing value of PGP in a short series. It enables the fan to separate the productive .300 series average from the unproductive .300 series average. One extra hit in 20 at bats separates the .300 hitting performance from the .250 hitting performance.