shift understanding

Good evening,I'm soon to be starting my first nursing job which i'm very excited about. I'm going to be working second shift but they also included a 72-72-80 mention on my offer letter. Could someone please explain that to me? I've never heard of that before.
It sounds like the first and second pay period (two weeks) you would work 72 hours and the third pay period you would work 80. I'm not sure how that would actually play out with scheduling though. Be sure to get the shift times and days clarified.