I make wine, and have found that the gallon and or imperial gallon marks on the buckets and carboys (the imperial gallon and american gallon are quite different also) mean nothing! I always have to use my usual gallon container and remark all the lines. I think the same is true for rulers. It depends on where they are manufactured, and whether they really care about precision. I am fortunate that my cutting mats are very accurate, so that is what I use for my measurements.
Phyllis