Depends on who it is. The more expensive, well established brand names tend to be more accurate than the discount brands. However, with the adoption of UN and IATA lipo shipping regulations I would think all the manufacturers have been looking for ways to ship larger quantities for less cost.
I can’t remember which brand it was, but about 3 years ago one manufacturer was placing a second label over the original, with a lower rating on the second label. People were buying larger capacity batteries only to see a lower capacity value on them after receiving them. Lots of really ticked off people until someone discovered the outer false label could be peeled away to have the right one. The act was intentional fraud to allow more batteries in each air shipment than the regs allowed.
You can get a lot more 5400mA batteries in a shipment than you can 6300’s. The allowable shipped quantity is cell amp hour limited. If one was looking to maximize profit to the last possible 1/100 of a cent, what might they do to obtain it?