I’ve written earlier about how to test the amperage of your various chargers but it never occurred to me that your total amperage depends not only on the charger you’re using but also on the cable. All USB cables are not created equally! Most cheap cables are not thick enough to handle a larger current so you’ll only get 500mah or maybe 1amp charging with them. If your phone and your charger both support 2 amp (or even 3 amp) charging, make sure you have a 28/24 gauge USB cable.
This post on reddit explains a bit more if you’re curious.