This is simply because that is how long it takes for the cashback to reach us.
Generally speaking, the merchants get invoiced for cashback at the end of the calendar month in which the transaction occurred. This means that if your transaction was at the start of a month, then you will probably be waiting longer for your cashback than someone who made a purchase at the end of a month.
After the invoice has been sent, it can take 1 or 2 months for the cashback to be paid to the affiliate network and for it to then be sent on to us.