I could be way off on this, but this might be due to the nature of how the collaboration works with Aaron; Aaron writes all the music and Taylor writes to track. True, she has to trace the melody, but I think it's substantially easier to be focused on your words and turns of phrase if you already have the music in front of you. It probably allows her to be more focused and poetic when she needs to be. With Jack, I think Taylor takes more musical ideas to him; more melodies, chords, etc. So she's having to figure out the music along with the narrative, and that's substantially harder. I'm not saying she never writes to track with Jack - she absolutely does - but I'm not sure I've seen an instance in the credits where Taylor write a song and Aaron produces it without having a hand in writing it, when I definitely think I've seen that with Jack (which just tells me she's doing more musical heavy lifting when she's with Jack than with Aaron). I hope that makes sense.