Perhaps it's an effort to encourage people, as Amazon does, to price at at least 2.99. In each case, the royalties are 5% higher than Amazon's, but obviously, 75% is much better than 40%.
I'm not currently wide, but if I were, I suppose the lower royalty would hurt during promotions, when I might drop below 2.99. Otherwise, the only things I regularly offer at below that are single short stories and one novella. I'm thinking of changing my price structure in any case, making prices in general higher, so that 2.99 would be the sale price. But I'm probably not going to do that until I'm sure the US isn't going into a recession.