Comes down to believing the mAh rating provided by the seller. I don't think anyone here has spent a lot of time buying laptop batteries and doing load tests.
They may all use the same form factor of lithium cells, but laptop batteries come in all kinds of shapes and sizes and chassis. If you are looking for more battery life, look for extended capacity sets. 6, and 9 cell packs are somewhat common for enterprise class laptops. Usually makes the battery extend outside of the normal size of the laptop.
Here is a 9-cell compared to the stock 6 cell:
http/www.newegg.com/Product/Product.aspx?Item=9SIA28U0TM0538