Strength of schedule is calculated from the opponents projected fantasy points allowed for the DL position. The best rating is +16 which gives the player the advantage against the defense and indicates an easy opponent, the worst rating is -16 which gives the defense the advantage and indicates a tough opponent.
Houston's 10 sacks over three seasons certainly isn't anything to write home about. However, Houston has averaged over 50 stops a season, racked up 69 tackles in 2012 and was an IDP DL2 last year in Fantasy Sharks Default IDP Scoring. Throw in that Houston will see the most extensive playing time of his career in 2013 and there's a real chance Houston will crack the top 20, making him a nice value given his current ADP.