The amount of current going into the battery is the amount of current the phone can draw from the charger (which is limited by the design of the phone, no matter how large a charger you use) MINUS the amount of current being used by the phone. Turn the screen all the way up, have all the radios running, run a few apps that all need a lot of CPU, and the phone is using more current than it can draw from the charger, so it's drawing some from the battery.
Does the phone charge as fast
when it's turned off as it used to? That's comparing apples to apples. If it is, the problem is your usage pattern and expectations. If it isn't, it may be a bad battery. (Letting the battery discharge down below 40% regularly shortens its life. Read
The Care and Feeding of Lithium Polymer Batteries.)