I didn't know the answer, but your question made me curious enough to try to find out.
https://en.wikipedia.org/wiki/Ice_beerIce beer is a beer style, in which beer has undergone some degree of fractional freezing during production. These brands generally have higher alcohol content than typical beer and generally have a low price relative to their alcohol content.[1]
The process of "icing" beer involves lowering the temperature of a batch of beer until ice crystals form. Since ethanol has a much lower freezing point (-114 °C; -173.2 °F) than water (0 °C; 32 °F), when the ice is removed, the alcohol concentration of the beer increases. The process is known as fractional freezing or freeze distillation.[2]