It's just the basics of how the airlines price fares. As stated more people search for flights on the weekends so fares can be several hundred higher than if you look on Monday.
If you watch a specific fare throughout the week it is very interesting to see where it goes. I'm looking at MSP-IAH-NRT-SIN-NRT-EWR-MSP with MSP-IAH-NRT and NRT-EWR-MSP on CO, that specific routing is around $1500 on Monday but by Friday it's 2000+ if it's even available.