I'm sure humidity does matter, although I'm not exactly sure how much it matters. Probably just a little. But you also mentioned that it was drizzling rain -- did the boil kettle get hit with drizzle during the boil? This could also have cooled it down, preventing normal boil vigor.
The wind can also have an effect, e.g., if wind is blowing the flame off to one side.
Also the amount of flame, e.g., amount of propane sent to the burner. Depending on your setup, it's not necessarily easy to get the flame at exactly the same strength every time you brew.
Perhaps the most significant factor, however, is the diameter of the boil kettle. You probably didn't change your kettle between batches, but if you ever do, it's obviously something worth considering, as you'll see... Forgive me for using a little simple mathematics, but if you usually use a kettle with a diameter of 18 inches, let's say, and now you switched to a kettle with 12 inches, this can have a huge effect based on the ratio of boiling surface areas. Since A=pi*(R squared), you could compare 9*9=81 square inches versus 6*6=36 square inches and based on that the boiloff rate for the 12-inch kettle is 36/81 or less than half as much as the 18-inch kettle. Quite a difference.