Why does my online job search engine behave this way?

[Aquarelle]

So I'm job hunting. I'd like to stay at my current organization (University of Minnesota), so I do a lot of searches on the U of MN's HR/employment site. But I'm open to leaving the U of MN altogether, so I also look at other search engines. When I search the U of MN's employment site, I have to be signed in as an employee. What I've noticed is that sometimes (often, actually), some job postings don't appear (at least for me) on the U of MN's own employment listing site, but I do find them in other search engines, such as the Higher Education recruitment Consortium (Higher Education Recruitment Consortium (HERC), HERC Jobs|Find Your Career Here).

Any experts know why this might be happening? Could it be because I am signed in as an employee when looking on the U of MN's site?