I think it also has to do with adding extras.
If I sell 12+, I provide 15 eggs.
If I sell 24+, I provide 30 eggs.
If I sell 6+, I provide 8 or 9 eggs.
So like others said, some of my breeds, I have 1 Roo and 3 or 4 Hens. I want to sell fresh eggs, so 8 or 9 is all I can provide. By selling as 6+, the consumer pays for 6 and gets a couple of extras. I guess we could sell 9 with no extras, but that does not sound as nice.
Does that make sense?