Why does one of my "middle" relievers face 1 batter in the 8th and then the other "middle" reliever pitches the 9th?

In the previous away series, my "short" relievers went 2.0 innings apiece to close out the last 4 innings.

Meanwhile...
Game 1 I had a big lead. Game 2 was a 2-1 ballgame.

Is it the time of the game or the closeness of the score that determines whether a short or middle reliever is used?

I don't mess with closers because I can't figure out when they get used (often they end up rusty for me it seems?). Are they only used in close ball games?