I have a few guesses, but may be wrong on all counts.
1. If your set is one of many, and hers is her one and only, your kit is getting more rest time to cool down and stay cool. Your wife may be using hers frequently and not giving it time to rest so that it stays a bit warmer and she runs more
juice through the system during the day.
2. If Veronica's mod is riding around in a purse, it may get bounced around more than your mod. You may have the same amount of brown building up in your kit, but hers is getting mixed and shaken to keep the brown distributed though all of the liquid in the
tank. Kind of like a vinegar/oil salad dressing.
3. I may be so far wrong that I'm outta this world. It is aliens. Yep, aliens, taking their invisible hits off your wife's kit and their alien breath is turning the
juice brown.