iphone 12 mini battery drains faster with apple original 20W than google pixel 18W charger

Hello guys;

I have been using Google pixel’s 18W charger for my iphone 12 mini since a year then I bought Apple Original 20W charger from an apple authorised dealer here in Pakistan and I realised after using it a couple of weeks that my phone’s battery is draining much faster with Apple original 20W charger than the Google pixel’s charger.

what could be the problem?