Rule 10.17(c) states:

The official scorer shall not credit as the winning pitcher a relief pitcher who is ineffective in a brief appearance, when at least one succeeding relief pitcher pitches effectively in helping his team maintain its lead. In such a case, the official scorer shall credit as the winning pitcher the succeeding relief pitcher who was most effective, in the judgment of the official scorer.

Rule 10.17(c) Comment: The official scorer generally should, but is not required to, consider the appearance of a relief pitcher to be ineffective and brief if such relief pitcher pitches less than one inning and allows two or more earned runs to score (even if such runs are charged to a previous pitcher). Rule 10.17(b) Comment provides guidance on choosing the winning pitcher from among several succeeding relief pitchers.

Good question. Perhaps the official scorer sees it as Tietje pitched well enough while the game was still relatively close, or that Grove did not contribute enough, or late enough into the game.

Posted by crazystengel on 6/25/2016 9:20:00 PM (view original):
Looks like the "5 inning rule" doesn't apply to relievers, at least at WIS:

https://www.whatifsports.com/slb/Boxscore.aspx?gid=26689647&pid=1&pbp=0&tf=12.95

Reliever Grove got the win in this one despite not lasting until the end of the 5th.

No idea why Grove would get the win in this game, and not in the game linked in the first post.

The decision is actually at the discretion of the Official Scorer in MLB. Since this would be impossible to do within the Sim, our process on how the Winning pitcher is determined is as follows:

A starter must go 5 innings to be credited with a Win. If the team has a lead and the starter does not make it 5 innings, the relief pitcher that is pitching at the end of the 5th inning will get the win. A relief pitcher will also earn a win if they are pitching when the team takes the lead that is not relinquished.