The capacity of a lithium-ion or lithium-polymer battery is determined by its physical size. There are no vendors out there with some magical process to create higher-density lithium batteries, so when you see a BL-5J battery announced as 1800mAh, you can be certain about
a) They are lying about the capacity.
And, due to a),
b) Why would you put trust in buying batteries from someone who are obviously trying to push fake stuff to people?

That there are smaller variations in advertised battery capacity is an entirently different thing. These batteries vary by some 5-10% or thereabouts, and, due to how conservative the advertising is, you may see the same battery advertised as 1050, 1100, 1200 mAh, for example. But if you see anything like a bump from the original BL-5J size of 1320 to -> 1800, then you KNOW they're lying. That's the kind of shops I don't buy from.

I did buy a replacement battery for my Palm with higher capacity, the original is 900mAh and the replacement around 1100mAh. But in this case the latter is physically larger and could only be (literally) squeezed in because it's lithium-polymer without a hardshell and it's possible to massage it into a shape that fits the limited room available.