This happens when you pay via PayPal and ask PayPal to withdraw the funds from a bank account. This is called an eCheque by PayPal.
PayPal will hold the actual payment for 5 business days despite sending an email implying that the payment has been completed. The 5 days is due to the Australian bank taking up to 5 business days to respond to the withdrawal request. (The extra time is due to the transfer being a "pull" request by PayPal, not the normal "push" transfer that we commonly do when transferring money.)
PayPal does not inform my system of the completed payment until it actually receives the funds and transfers them.
SHORT STORY: If you pay via PayPal using a bank account (instead of a credit card) then the subscription can take 5 business days to become active.