SLT Unlimited Family Advance Charge?

I recently upgraded to the Unlimited Family Package from SLT for 7900/- without tax.

But I have been charged for the month of February in the January Billing cycle which is unusual.(See attached bill) Called 1212, agent says this package has some advance billing which I’ve never heard before. I sent an email to 1212 regarding this but was wondering if anyone else faced the same issue.

TIA