Exposure calculation sky effect

[mwg3656]

When I check the exposure calculation time there is an inconsistency.

Same Imaging system for both, just date change.

Full Moon April 7th, target SNR 30 calculations
- M81 mag 7.8 60min (looks ok)
- M1   mag 8.4   8sec (not ok)

New Moon April 22, target SNR 30 calculations
- M81 mag 7.8 60min (??, time should be less with new moon)
- M1   mag 8.4  8sec  (still too short)

1. Does sky limiting brightness (19.6mag/sec) set these exposure times?

2. Why is M1 so short?

Mark