-That is because USCIS made the mistake to issue you one year EAD (they were supposed to issue your two year EAD). Then, you asked them to replace it for the two year one and of course, you did not need to pay the fee again for the repalcement. If you renew it after the two year EAD expires, you have to pay the filing fee again and again very time you renew it.
Of course you must apply it in 90 days before the old EAD card is expired. What i am saying is that all you need to pay your EAD card just once it won't be charged anymore for any other renew.
-Not true unless your I-485 filing fee was $1010.