There are indeed different capacities.. I suspect there are also different grades, but I can not prove it.
I do know this, Many battery makers make batteries with like a 90 day warranty, a 2 year warranty and a 5 year warranty, All of them have specs that are dang near identical.. But they have different warranties.. One assumes the design life is 2x the warranty.. But if they are all identical.... Then it would make sense to only buy the 90 day wonders.