The charger supplies a fixed amount of current. If the phone uses more than that amount, using it while it's charging results in a net discharge. (If you regularly discharge the battery past 40%, you'll kill it quickly - the more often you do it, and the deeper you discharge it, the faster it dies. Lithium batteries are not deep-discharge devices, they're lightweight devices. You don't want to be carrying a lead-acid battery around with you even though it can take constant deep-discharging.) You may need a new battery.