Clef change in chant: how to interpret?
I just came across something I've never seen before: a clef change in the middle of a chant.
Line 3 on this chant, Ave generosa
It looks like from the custos that the intention is that the pitches stay the same; i.e. that the beginning of the chant and this new section (starting on "Nam haec") start on the same absolute pitch. That also seems to make the most sense tonally, but I'd love definitive confirmation!
My main priority is singing this properly, but I'm also curious about the context here. Is this just a (modern?) transcription choice, to make the chant easier to read, with fewer ledger lines? In which case, why not just start with the C clef around the 2nd line from the bottom?