Exit versus Close

Good afternoon.

I am having a bit of a conundrum when working in Storyline on a project.  I have completed a course and, originally, I had it set to successfully complete the course by slides viewed (15 slides must be viewed out of the 15 slides in the project). There is no branching in this project, and it is linear - a user cannot click ahead and can only sit and listen to the audio while the slides do their thing.  When I inserted the course into our LMS (it is the LMS by Grace Hill) I can view the course, and I can go all the way through, but it does not mark it complete once it is over.  I went back into the project and added the "Exit" button through the Player options.  When I added the Exit button, I can now go through the entire course, and click "Exit" and have it mark the course as complete.  If I run through it and still click the red corner X to close, it keeps it marked as incomplete.

Can someone let me know what is going on - why is success being marked by the Exit button (and successful viewing of all 15 slides) versus just the 15 slides being viewed?

Thank you for your time.