I'll try to explain it briefly..
Field strength is broken down into three catagories for FM signals: 70dBu, (City Grade), 60dbU, (usually a good signal and protected contour), and 40dBu, (protected zone depending on class of station and can be considered "fringe" coverage). FM, (VHF) frequencies are received line-of-site. So, as with KXXO's existing facility, just because North Seattle is in the 40dBu or less field of KXXO, you can still hear it quite well in a car on I-5 due to the terrain "slot" along I-5 in relation to Rooster Rock. However, because the field strength is so low due to distance, the signal can't penetrate buildings, so in-home or office listening is very limited.
The interesting thing about the stations who have located to the South West Olympic foothills, is the challenges with terrain blockage to the North East to such key diary return areas such as Kent, Auburn, Maple Valley, Bellevue, etc. Essentially everything East of I-5 is toast due to the hills and valleys, no matter what the calculated field strength is. KXXO's move must be for something other than improved coverage toward Seattle, because based on measurements and observations of the stations who have built on the SW hills, those stations don't have very good coverage into the Seattle Metro as a whole. Chances are however, KXXO will still play fine into Olympia, so if that continues to be their main focus, then it should be fine.