If you start out even and stiffen the back bar the back will become lose and slide out. Yup.
If you start out even and stiffen the front the front will plow Uh-huh.
Which ever end is stiffer will lift the inside tire more and make the point of the tricycle on the outside If the spring rate and wheelbase remains the same. Neither of which apply to this particular situation.
Side note: By moving the rears outward, you're changing the angles of that very triangle when looking at both wheels on either end and one wheel at the other. (the further out you go at the rear, the closer you get to a right triangle. The closer to right triangle you get, the easier it is to lift the odd wheel out as there's less leverage against it. In the case of an unmodifed suspension that's had only rears moved outward, that easiest to lift wheel after lading becomes the inside rear)
Away from all that and back to just springs and ARB: The spring rate will act different by some proportional amount as the same leverage change applied to the ARB as a result of moving the wheels in or out effects them too. (a longer lever is a longer lever to everything it's applied against)
Loosely put, if shoving the wheels outward makes the ARB acts softer due to more leverage against it, then the springs will also act softer.
The 'softer' springs would then be more in sync with the 'softer' ARB allowing for the greater leverage against the bar and spring to lift the inside tire through the extra mechanical advantage. (remember, it's not changing bars or springs we're talking about, it's changing the leverage used against them)
LOL
Ok, all I know from practical experience is that moving the rears outward and leaving everything else alone will loosen the rear a bit and that the inside rear is slightly more prone to lift in a high roll ~ high traction situation.
In a low roll ~ low traction situation it runs more in the neutral to neutral loose range, but it sure doesn't *understeer.
* unless you go in with one front wheel hazing on the brake, then you might get enough push to where you think you're plowing a driveway. (oh, yeah, and hope that there's no type of crest, drop, ripple or uneveness of the driving surface at the apex, or you'll find out just how loose it can really get as it unloads itself in a rather shocking fashion

)