In a chemostat the OD is limited by the nutriant content of the medium being pumped in.
LB is not a defined medium therefore the limiting nutriant may vary in concentration so the same portions of ingridants may lead to diferent ODs so to get round this we make an LB+Glycerol medium. So the glycerol is the main carbon source and the limiting nutriant. The LB simply provides other salts and ions.
A second crutial diference between the chemostat and the batch culture situations is that in a batch culture the cells are showing unlimited growth and growing at their maximum rate. In a chemostat there is one limiting factor which is slowing the growth rate of the culture. If this is a carbon source (such as glycerol) then the cells will be limited by the amount of energy they have avalibale and behave in a similar way to the unlimited cells (but Slower)