Why do college football rankings change?

A few games last week among top-ranked teams.

#2 Tech (#2 : 0) beat #9 Oklahoma State (#13 : -4)  56-20

#8 Utah (#7 : 1) beat #12 TCU (#18 : -6)        13-10

#7 USC (#6 : 1) beat #21 Cal (Unrated : ->4) 17-3

unrated Iowa beat #3 Penn State (#8: -5) 24-23

In all the games, the home team won. Three of the wins would be expected, with the higher rated team winning at home. The Penn State game was an upset, though it was close. Did TCU suddenly become a worse team because they barely lost to a team that was better than them?  Oklahoma State got blown out by a higher rated Tech team.  But does this just show there is a bigger gap between the top teams and the rest?

Even more baffling is the relative difference of the changes.  Why did TCU fall 6 spots after barely losing on the road to a higher ranked Utah, while Penn State only dropped 5 spots while losing to an unranked Iowa?  And why did Cal fall so far after losing to a much higher ranked USC?

The seemingly excessive gyrations help explain the cupcake non-conference scheduling. A loss to a better team hurts a lot. Beating an inferior team helps a little. Thus, the goal is to avoid better teams as much as possible, and schedule all games where wins can be expected. 

Perhaps it would be better just to do away with this BCS garbage and "pseudo championship" and go back to the old bowls as bowls.