They are smaller than a gallon…
I’ve spent some time working on pot sizes… most of the time, they are not what they say… …except for 3 gallon… 
I went through a bunch of similar pages and ended up adding a sheet to my master spread sheet - just for container sizes… just to contain this weirdness… 
The ‘driver’ for all of this was to be able to calculate potting soil needs…
{1 gallon (US) = 231 cubic inches}
Cheers
G