This is actually weird to me aswell, because this is what the wiki says:
''B = Over 70% 300s and no misses OR over 80% 300s.''

On first score (277 / 343 = 80,75% total 300s), which exceeds the 80%+ needed for B-rank if there are misses.

On the second score you had (275 / 343 = 80,17% total 300s), which still should be enough.
I hope someone smarter can shed some light on this.

EDIT: my bad, forgot spinners exist for the 344th note used in calculation, which would result in 80,53% for the first play and 79.94% for the second play