First, all the TV stocks are for shows on the "Broadcast" networks, the Big 4-1/2. (ABC, CBS, NBC, FOX, CW) The premium or pay cable networks don't operate the same as broadcast or even the "basic" cable channels (like USA, FX, TNT, etc.). The broadcast networks order a series, usually with an initial order of 13 episodes, but there is not guarantee that they will actually broadcast all of them. If the ratings are poor enough (and therefore the ad revenue) it will be removed from the schedule, if it does well enough it will get an additional episode order. Most of the time when additional episode are ordered it is what is called a "back 9" order to complete a standard 22 episode season, but not always. So there is real uncertainty about how many episodes of a new series will actually air.
Basic cable usually has fewer episodes per season and is also less reliant on ad revenues since they get carriage fees from the cable companies. So they will almost always air all the episodes for a given season since they frequently have not only ordered all the episodes but have finished filming them before the first airs.
Premium or pay cable (HBO, Showtime) don't have ads and aren't reliant at all on ad revenue. So they don't even really pay attention to ratings, let alone pull a series off the air for having poor ratings.
Hope that helps and welcome to HSX.