As previous stated, "why" may not be important to you, but the OP has his/her reasons. Not to mention the "standardization," etc. It's also irritating (for an OCD personality or for someone with System Design experience) for something to behave differently under random circumstances; unless a particular reason can be sited explaining why, of course.
In my case, I don't like having the dimension line sticking way-out from the related object - prefer it relatively close to the item being annotated. If you can't change the side of the callout, you have to place the callout further out, even if just to avoid the item dimensioned itself or overlaying constraint indicators.
Also, JHackney's solution worked for a "stand alone" dimension, but not for a "constrained" dimension (might be another reason for the difference, ...) In that case, reversing the start/end reversed the distance sign (- vs +), but did not switch the side of the callout line.
Apropos nothing important, how do you capture your Fusion screen as a video? Might be useful tool if (when :)) I get to the point where I'm in way, way, over my head. LOL Right now I'm just getting into learning the platform. I just love wildly interrelated, complicated, programs. I used to specialize in designing and coding them, at least with respect to data reporting, efficiency and work flow, not imaging or CAD.