At least from personal experience, most Apple Retail Stores didn't seem to exhaust their supply on Friday the 14th. So having more units on hand (and the usual second round of deliveries on Saturday), plus the pre-order records contributed to breaking this record. Simply having the product available was probably the biggest factor, though pre-orders (confirmation tomorrow) do indicate that demand may have increased over the iPhone 4, although some of that that could be seasonal demand.
I'm not sure on carrier counts (iPhone 4 not known, iPhone 4S launched on 22 carriers). In the US, the iPhone 4 only launched on one carrier, whereas the 4S launched on three. That's also going to be a significant contributing factor in increasing its effective availability to buyers.