Maybe they lived there prior to purchasing the home?  I wouldn't pay much attention to the address, although it does seem a little odd.  Federal Tax Liens attach to after acquired property, so did nobody check that when they purchase the property?  Just curious, but was there a purchase money mortgage?

I don't know anything about the Commonwealth liens, since I am not in Pennsylvania. 

Kudos to you for picking up on that.  Not enough title abstractors would even notice... or care. 

Best,

Robert A. Franco
SOURCE OF TITLE