Generally, weekdays are more expensive than weekends at business travel hotels. Often times, if you're staying on a Sunday through Thursday, you'll get the weekday rate. If you're staying Friday & Saturday, you get the weekend rate. And if you're arriving Thursday, staying through the weekend, you'll get the weekend rate on Thursday. And if you're staying Sunday night, but coming in prior, you'll often times get the weekend rate on Sunday as well.

As others are noting, in your situation, it may be better to make 2 reservations and then just let them know at check-in that you've got a second reservation & they'll combine them and let you keep the same room then. It's probably just a systems glitch that's not intentional.