[dasil003]

North Dakota and Minnesota are colder than Montana (at lower elevations too). Take a look at the climate data for Fargo, ND for instance, significantly colder than Calgary. It's colder because it's more continental, and you often see a big bubble of arctic air dipping down on the weather map.

[dreamsofdragons]

Calgary is a bit of an outlier, they get a weather phenomena called chinooks. It's not uncommon for them to get several periods throughout the winter where their temperatures are above freezing due to these chinooks.