The penetration to the basement _may_ be because of the central stairwell connecting the three floors. It's not like the signal has to penetrate three concrete floors. The signal probably bounces of the various walls until it makes its way into the basement... or I have just revealed my profound ignorance of how router signals are propogated.
Happy Thanksgiving.
“Happiness is having a large, loving, caring, close-knit family in another city.”
- George Burns