Logic to select supplier from Sourcing rule

I would like to know what is the logic to select a supplier from Sourcing Rule.

I have a sourcing rule with 2 supplier and allocation as 50% each and rank 1 for both. Both have active ASL, assigned to valid quote as sequence 1. When I create a PR manually, system is always picking one supplier. I checked the quote date, created a new one with lower price and assigned to the other supplier, even after system keeps selecting the same one.

Any idea what is the logic behind it?

Thank you,

Gislaine