The reversal of the order—from Ammonites and Philistines to Philistines and Ammonites—appears to be purely for stylistic reasons. The chronological order remains Ammonites first (the Jephthah narrative) and Philistines second (the Samson narrative).1
7 So the anger of the LORD was kindled against Israel, and he sold them into the hand of the Philistines and into the hand of the Ammonites,