The back of the $20 (http://en.wikipedia.org/wiki/File:US_$20_Series_2006_Reverse...) looks like a child took a highlighter and drew 20 on the bill dozens of times. There's no discernible pattern; it looks like noise.
The front of the $10 (http://en.wikipedia.org/wiki/File:US10dollarbill-Series_2004...) has three things going on in the exact same place. Look at the seal on the right of the bill. It also has TEN written over it, and We The People in the same spot. Is it harder to counterfeit if you can't read it? It simply looks lazy to me.