Terrible service from FNB

I was charged R48 service charge for a withdrawal of R3000 from ATM on 18/12/23. I was under the impression that this fell into my free withdrawal amount but was surprised when I queried on secure chat that it counted as 2 withdrawals (1 for November and the one mentioned above) due to the fact that FNB bills me for bank charges on a 18th to 18th cycle. I base my usage on a calender month as that is how my earnings work, how is it that the bank just decides as they feel like, I am a long standing customer and it seems that the bank is taking advantage of unsuspecting customers by looking for loopholes and grey areas to charge extra. Please get back to me as I do not accept the fact that the bank bills for bank charges from the 18th to the 18th, if i was told about this beforehand I would have opted for a calender month billing (this is what I had always assumed I was being billed at hence my usage of the account to not attract additional charges). Highly disappointed FNB.