Because most chargers can't discharge that high on higher cell count lipos. The really small ones like the 50W ones I dont think can even do 5A on 2S packs.
3A isn't that bad really, the discharge power won't be as high as the charge rates will be. Unless it's a high powered charger that has the ability to go higher.
That's how they market things, It's trickery with words.
They take the lowest sometimes second lowest cell count and give the max it can charge or discharge with that lipo. What they normally give you is the power and it leaves the buyer to figure it out using the power triangle. click for calculator