It seems to be because they are below passing, but not all below passing scores have them in different gradebooks.
This is a setting controlled by each teacher in their user settings, I think. https://support.agilix.com/hc/en-us/articles/204968166-How-do-I-change-my-user-settings-
I think this is a visual indicator of the failing grade in addition to the color. It's designed to help those who might be color blind and can't see the red shading. See if that makes sense.