Why does PayPal Credit not pay the oldest?

Description

I’ve made a couple purchases using Paypal Credit and I understand I cannot select which gets paid back, but why is it always selecting the newest one?
Here is an example, I make the following 3 purchases with the following expirations:
– $400, exp 21st Feb
– £200, exp 21st March
– £200, exp 21st April
If I pay £400 into the credit to pay it off, why is it paying the newest ones (2 and 3) and not the oldest one (1) which will end up gaining interest?
Is this by design to squeeze as much money out of people and never allow them to actually pay it off, has anyone else noticed this or give some advice?