Teams often ask me why their seemingly lopsided win is costing them points.
TLDR: In order to have a game be thrown out as a blowout you need to meet three criteria:
The winning team needs to be rated at least 600 points higher than the losing team
The winning team needs to have a least 5 other results NOT being thrown out. In cases where a team has more blowout eligible games than it can throw out (for example, 5 blowout elgible wins in 8 total games) the most damaging games are prioritized
This is the tricky one: for the game in question, (winning_score) > (losing_score*2 + 1). Don't forget to apply order of operations (multiply before adding) AND notice that it's a strict "greater than". So a 13-6 game would not evaluate to true since the 13 > 13 is not valid. See the table above. It is possible to get the "max point" score differential value without the win qualifying as a blowout (yellow squares on the table).
Additional context for the less familiar:
Your ranking is a weighted average of all your individual game ratings. Playing a game that rates above your average helps, playing below hurts. The rating for an individual game is calculated as: [opponent rating] +/- [goal differential value]. In that expression, the +/- is determined by if you won or lost and the "goal differential value" is a function of the point diff of the game that ranges from 125 (a DGP game) to 600 (a game in which you score more than double your opponent's score). The table above charts the goal differential value for different game scores.
Given that framework, you can imagine that there are technically games that you could enter into where you are unable to achieve a game rating equal to (or more than) your rating average. If your opponent is more than 600 points below you, even if you got the maximum goal differential value (600) the game rating would still be (opponent rating + 600) which by definition is below your average. At its surface, this game can only hurt you. But...
The "blowout rule" exists to help avoid this issue. It doesn't let you earn additional points but it does allow that game to be omitted from your average. However, the threshold for it is harder to meet than the max point win above. Additionally, the criteria is often misunderstood. In order to get a game ignored, the winning score has to be MORE THAN (2*losing_score + 1). So in a game to 13, the first blowout eligible score is 13-5 not 13-6.