Default Lands

I'm not sure if there's somewhere this is documented that I can't find or what have you, but what decides which lands Forge chooses as the defaults (for instance the lands it gives you in quest mode)?
I'm also confused by something that happened, and I would appreciate an explanation if someone knows what the story is:
I finally got around to finishing upgrading my card art to higher resolution versions, and one of the ones I was missing was M10.
I made new versions of M10's basic lands as full-arts since I'm looking at them all the time. I couldn't find good sources for some of the lands so, for instance, I replaced two of the forest images with my new versions and figured I could just get rid of the other two for now. I was expecting it to just show those two pieces of art for the forests, but instead some of them started showing up as CHK forests. I was wondering why, in particular, the "default" lands are M10 and why it would pull alternatives if there are only two variations and, more so, why those alternatives would come from CHK of all places? I'm also wondering if there is some way to easily change which lands are the "defaults"?
I'm also confused by something that happened, and I would appreciate an explanation if someone knows what the story is:
I finally got around to finishing upgrading my card art to higher resolution versions, and one of the ones I was missing was M10.
I made new versions of M10's basic lands as full-arts since I'm looking at them all the time. I couldn't find good sources for some of the lands so, for instance, I replaced two of the forest images with my new versions and figured I could just get rid of the other two for now. I was expecting it to just show those two pieces of art for the forests, but instead some of them started showing up as CHK forests. I was wondering why, in particular, the "default" lands are M10 and why it would pull alternatives if there are only two variations and, more so, why those alternatives would come from CHK of all places? I'm also wondering if there is some way to easily change which lands are the "defaults"?