No one can say for sure why that is, but it’s most likely because most Cal Poly grads work on the west coast, where salaries (and the cost of living) are very high and most Purdue grads don’t. My son has multiple friends who are CP CS/SE grads, and two years out, they all make more than the median number reported on College Scoreboard. They all work for CA based companies, but they don’t all live in CA.
I bring it up because as far as I know it’s the only source of non self reported salary data.
It’s pretty obvious thought that, as is the case with most schools, CA grads stay close to their alma maters. CA salaries are higher than the rest of the US for most engineering disciplines. Take it all with a grain of salt, objective or not.