The numbers on the y-axis have to be power rankings. Ignore the "-". If you look at Denver, they hover around #1 most of the season. The Seahawks are in the top 3/5 most weeks. I'm not sure whose power rankings they're using, though.
On those graphs, there are two teams that have been quite consistent - Seahawks and Broncos. Look at the graph for those teams compared to others, I believe it's quite telling.