Toyota's battery-charging algorithm is probably very complicated in order to correctly maintain its state of charge (SoC) inside their chosen range without suffering from any memory effect. Their patents on this are very detailed, and discuss many ways of monitoring and controlling it. ken1784's very interesting link shows that (at least for the Prius, and undoubtedly also for the TCH) the SoC display is decidedly nonlinear, in that fully 57% of the usable SoC is represented by bars #6 and 7 alone! No wonder the SoC display seems to spend most of its time in this range. This is probably also deliberate, so as not to alarm drivers by seeing the SoC continually going up and down. (This is rather like what they have done with the temperature gauge, which sits at the same reading for water temperatures from 60 to at least 90 degrees C.)

Stan