I've never heard of an acid carboy until you mentioned it, but anyway vessel openings can be highly variable in size. I don't think manufactures really care too much about variances of the inside diameter of the neck in glass vessels.
For example, I have two 5-litre brown glass jugs made by the same company with the same marks, etc. The screw lids are interchangeable, work just fine. However, I had to buy two different size bungs for those jugs at the LHBS because one of the openings is smaller than the other one and the bung could very well have fallen in.
Originally, I had bought two same sized bungs and noticed the problem when I got home. I measured the inside openings with my digital micrometer, returned one of the bungs to the LHBS and told him the measurement. He pulled out his bag of bungs and his own old-school micrometer, then gave me a new bung. It worked perfectly.
You can get a cheap little digital micrometer for less than 10 bucks. They are not used everyday, but when you need a precise measurement they are an invaluable tool to have in your collection.