"Then why is it that a mother who is pumping for her baby while she is
employed must leave more and more milk in the bottles?  Is this happening
because someone believes it should happen?  Or is it all so different when
it doesn't come from the breast?"   

My feeling is that babies drink more from bottles, be it EBM or ABM, then they actually get at mom's breast.  I think it is  due in part to the fact that the milk comes so fast, compared to straight from the tap.  First, it comes too fast to do the nurse, pause, CCK release,
(am I done yet?") then nurse again, that happens to the baby at breast, so he stops when he's finished.  Also, it comes too fast, I think , for most babies to satisfy their sucking needs.

Then babies get used to the increased quantity, and need the overfull tummy as well as the CCK to tell them they've had enough.

Take a look at the brandnew born.   Baby might nurse for 40 minutes for what is essentially 5-10cc.  But if you give him a bottle, he seems to need at least 15-20 cc, even of EBM, before he settles down.  

I do notice, however, if we give the baby the same 5-10 cc very slowly, letting him suck it off a finger, or with a syringe, a few drops at a time, then he doesn't seem to need as much volume, and will often take very little, if any, formula on top of it.

Chanita, San Francisco