just two assumptions:
#1: Is your head with snorkel as relaxed and you're looking down same way as without? If not, this might cause 1-2SPL more easyly...
#2: I don't have a snorkel, so my active rests are most times "froglike backstroke with relaxed head and without using arms" because this is totally different to FS and pure relaxation (for me) without any breathing issue. It shows the start for the next round feels easier than just standing at the pool's egdge for same time.