It all depends on supply and demand.

If you have 6-8 months left in your lease, and they do not have demand, they got you locked in so their incentive is less. They do not have demand so obviously the unit you vacate will also sit for a few weeks at least so you need to pay. That's the contract.

If you have 2 months left, they know you are high risk to move out so they will throw discounts to avoid yet another vacancy.

It's possible the parent only had one month left on their lease.