Why does water appear just below my basement slab in one location and not another?
Yes, gravity exists under your house and I’m pretty sure Sir Isaac Newton would agree, but I don’t think he could explain this...

But here are some ideas: 1) a small underground spring, 2) a bearing wall between the two locations, 3) Major addition separates areas

1) Because there’s different soil conditions at the two locations, it makes me think the original builder knew of a small spring and installed some gravel and a discharge pipe. The pipe is located about 12” down so the water never gets too high. 

I’d check around your house and see if a pipe extends to a ditch, street gutter, etc. (Follow the roof drains too.)

2) If your house has a major wall separating the two areas, it could be keeping moisture from infiltrating your new hole. 

3) Likewise, a major addition could have added a new foundation area, which is separate from the wet area. 

I’ll check with Newton and see if he has revised any of his theories regarding basements...and then get back to you.