That looks to be a lipped and veneered door. Two things could be happening here. One is that the fading, or slight bleaching that naturally occurs in sunlight ,has been removed by your sanding revealing a darker wood. The other is that the veneer layer is now so thin , that the oil has soaked into the substrate below, giving rise to the discolouration.
The previous poster has just pipped me to it, but the solution is to evenly strip the whole door and re coat it with your finish. It won't alter the colour but at least the doors will be even.
I always found that using a scraper, or scraper plane was a lot easier on lipped and veneered work, as you can count the number of shavings you can reasonably take off, before you reach the ground beneath. Sanding, doesn't necessarily give you this fine tuning