Neither. It’s not a glitch, but it’s also not always true. If you take a look at your screenshot, note that the United flights are showing up with UA flight numbers. The UA flights — even though they’re the same plane, operated by CM — have distinct inventory, and inventory guides pricing. Sometimes this can work in your favor, sometimes it’s neutral, and one times it works against you. Unfortunately, for your dates, it looks like UA’s inventory is more restrictive, so the prices are higher.