It is possible some fly-by-night battery retailers exaggerate the batteries true capacity. But there aren't very many battery manufacturers and all the big ones are on average pretty straight-up in their ratings. Verging on quite conservative for the premium producers. Generally, as long as you go with a reputable battery producer you will get what you pay for. Often more than their stats say.

Some of the confusion is that people think batteries are like buckets, you get out what you put in. You never do. Even if you could you really wouldn't want to. Once the voltage on the battery drops your ability to get the energy out in a usable form comes to an end. The battery still had energy remaining but not enough to do anything with.

Also, before the voltage drops below usable levels the battery is getting worn down. The lower you take the battery the fewer cycles you can get out of it. The rule of thumb is that only a third of the batteries capacity is usable if you want to get the maximum life out of the batteries. All the reputable battery manufacturers are quoting the accurate figures on what the batteries will do. it is up to you effectively use what you buy.

One of the best manuals for setting up a system comes from the world of boating. Most libraries can hook you up with a copy of:

"Boatowner's Mechanical and Electrical manual" by Nigel Calder.

he has an in-depth discussion of various systems, batteries, chargers, and how to put them together to do what you want.