So this is a weird one.
3 weeks ago I made an Xmas beer. I brewed it, pitched the #2 yeast, and put the carboy in my cold dark room (about 70 F) to ferment. The fermentation and krausen began about 18 hours after brewing, and lasted for 6 days or so. I measured the FG and it was spot-on so I continued as planned. All was well.
I really like the recipe so I brewed another batch 2 days ago (on Sunday). I pitched the #2 yeast and again put the same carboy in the same dark room at the same temperature. Again, the fermentation / krausen began about 18 hours after brewing. However, tonight I took a look at the carboy only to find that the krausen has completely sunk to the bottom... 4 days earlier than the identical recipe did the last time.
What happened? It's still fermenting given that I can see the 3-piece airlock bubbling, but what is the deal with the super early krausen drop? I'll give it 7 days, measure the FG, and hope for the best, but can anyone explain the reason that batch 1 went 6 days before the krausen dropped and batch 2 went less than 48 hours?!