My thoughts, as a "pumping veteran":

It could be that the milk appears different on both sides because the
mom may have nursed her baby last on one side.  Therefore, one side may
be into "hindmilk" and the other into "foremilk", so they may appear to
be different composition.  Personally, this sometimes was the case for
me, and also I never got the same amount of milk out of both breasts.  I
would often get more out of my most favored nursing side, except if my
baby nursed there recently.

Cynthia Hodges
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