The one bad assumption people make about 1-run records is that a good late inning bullpen crew should automatically lead to a better record.
Sure, the good bullpen will protect more 1 run leads and give you more 1 run wins that way.
But a worse bullpen may start the 9th with a 2 or 3 run lead and given up a run or two, so you win by 1 run. So a bad bullpen can also lead to more 1 run wins. The Orioles bullpen failure Saturday night gave us a 1 run win which improved our record in 1 run games. If Kimbrel had been lights out, we would have had one LESS 1 run win.
1-run wins aren't just games where you lead by a run and hold on because your bullpen pitches well. They are also games where you lead by more and your pitching allows the opponent to get to within 1 run.
Similarly, 1 run losses are not all the result of bad bullpen. Maybe you are a run behind in the 7th, and you lose by 1 run because your good pen doesn't let the opponent add any more, and you don't score. Whereas a worse bullpen, you might have given up runs in the 8th and 9th lost by > 1 run. So you get an extra 1-run loss on your record BECAUSE your bullpen is good in that case.
These situations all muddy the water when it comes to 1-run win%. There is not necessarily a correlation between bullpen strength and 1-run game win% because of all the different ways a game can wind up a 1 run game. We tend to focus on the single case of "1-run lead in the 9th" where having a good bullpen will obviously make your 1-run record better. But those games are actually a small % of the cases.