I think the difference is whether you are taking about single-user pumps or hospital-grade pumps. As I understand it, the hospital-grade pumps all have some mechanism to prevent the backflow of air/other substances into the milk from the pump casing. 

The single-user Medela pumps do not have this mechanism - the Ameda ones do (by virtue of the design with that silicone cup that sucks in and out). Every pump casing is at risk of getting junk in it as the air has to be sucked into from somewhere - I've heard pretty gross stories of all manner of things finding their way into rental pumps (oh the war stories lactation consultants tell :) ). 

HTH,

Michelle

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