(edited after going through a buy)
I see what you mean, the 12 mini 64G with no carrier is 629, the carrier ones are 599 if you try to buy one.
Anyway the fine print is here, right on the page where you select the carrier or no carrier:
Connect to a carrier now.
Connect to a carrier through us now and your iPhone will arrive ready to go. All iPhone models are unlocked except those purchased with AT&T Installment Plans.
Also I noticed that the model number for Canada is different than the one sold in the USA, but I can't figure out what the difference is.
https://support.apple.com/en-us/HT201296
-David