I have no stake in the Florida situation, but a close look at the last 3 situations says matchup scenario to me.6/29
Meyer starts the 9th in a save situation to face the lefty heavy top of the National's lineup. Nunez comes in with 2 down to get Willingham out to end the game.6/30
Rain shortened game. Nunez warming in the bottom of the 7th for the 8th inning.
The last Nationals hitter to bat was Adam Dunn, so Nunez was being warmed up to once again face Willingham and the National's 7-8-9.
(My take from that scenario is that if there was a save opp, Meyer would have then been used to face the top of the National's order in the 9th.)7/1
Meyer enters in the 8th to once again face the top of the National's lineup consisting of mostly lefties and switch hitters.
So that's 3 straight games where Meyer was brought in to face the top of the National's lineup (essentially W.Harris (L), Guzman (S), N.Johnson (L), Zimmerman (R), and Dunn (L). To me it seems like Freddy is going based of matchups. Consistently giving the left handed heavy top of the order to Meyer, and the post-Adam Dunn bottom part of the order to Nunez.