Screen Brightness on Sprint vs T-Mobile Variants

[RG129]

I'm currently evaluating T-Mobile on a GS4 while still keeping my GS4 on Sprint. I've made both phones exactly identical and when placing the phones next to each other, the T-Mobile phone sometimes seems to have a dimmer screen. I'm assuming the light sensor isn't as sensitive and doesn't want to increase the brightness. Other times, the screens are virtually identical.

Is there something I can do to adjust the light sensor sensitivity? Why wouldn't they behave exactly the same? (I still prefer to keep auto-brightness on as it prevents me from fussing with the brightness throughout the day).