It changes the % allocated because curation reward is dependent on stake. If a good amount of stake donates curation rewards with votes before 30 minutes, it means more curation rewards in total will be donated to the post author.
What is the purpose of this rule? I don't get it.
Curation rewards should be a fixed percentage of the total payouts and people who find good content early should be rewarded but i really don't see the point of the rule you mentionned?
Good curators should be able to make a living of curation..with many post getting 3% curation rewards no one but bots is going to invest their time curating.