There are 2 mechanisms working at the same time. 1 of them is the reversed-auction you mentioned, which means that the first 1800 seconds after publication every second your potential curation reward grows by 1/1800th.

The second mechanism i mentioned makes sure that not everyone can just wait for 30 minutes to get maximum rewards.

I'm 99% certain this is the current implementation. If you have any proof to the contrary i would love to see it :)