It's not done on purpose. You need to remember that the show is filmed over 8 days and the scenes aren't shot in order, so those two scenes may have been shot 2-3 even 4 days apart.
They do have a continuity co-coordinator on set, who's job it is to keep track of things like that, but sometimes there's other issues which come in to play, such as mechanical break down, or that particular jeep being used to fill another scene at the same time (it's often normal to have 2 filming units working in parallel, especially on a tight 8 day schedule). In such cases they simply don't have the time to reschedule filming of a scene, so they just have to MacGyver it.
