My theory is that they make a certain number of offers of each level available, and reload occasionally. I had no problem getting a 75k offer when the link was initially posted, or when ppl started seeing the 75ks again. There was a point in between where I only got 25k and 50k offers, and then it seemed to be mostly 25k. Last week I got a 75k in private browsing on Firefox pretty easily, but today seeing only 25 and 50k again.