So, IF (and that is a big if) the last two checks were correct, baby gained an ounce in 24 hours. Right? So that is good....But unfortunately, whenever a different scale is used, it is a toss up of if the check is accurate.

How much are you expecting to pump at a time? If your baby is nursing that much, pumping one ounce at a time is very good pump output. I do not know why you think you need to save up two ounces to give baby at a time, I mean you can, but there is no need to do so. See more about pumping here:

Of course usually when a baby is not gaining well, it is because baby is not getting enough milk. But those babies are not pooping and peeing within normal parameters, nursing with normal frequency and length of sessions, transferring milk very well when it is measured, and appearing healthy and active. And your baby is. So??? It is a head scratcher.

I really think there are two possibilities here. One is, monstrous scale error, starting with baby being weighed totally wrong at birth, probably. You have not seen the pediatrician since day 6. Is the lactation consultant in the same office using the same scale as the pediatrician?

The second is, a medical condition.