Why didn't it freeze??

Last night I put 4 bottles of beer in the freezer which is set at -20 C, to give them a quick chill. But forgot about them then and went to bed. This morning I took them out of the freezer and 2 were frozen solid but the other 2 were still liquid beer. Not a sign of any slush or ice at all. They were all obviously very cold as condensation forming on the outside of the bottles, froze immediately.

So any ideas as to why 2 froze solid and 2 remained liquid? I can't face a taste test today, but maybe tomorrow!


editing to mention that I am in Ghana and it is Ghanian beer. Quality control mightn't be the best here so I wonder if the unfrozen beer could be of a much higher proof. Wish I had a hydrometer!