Water merges onto pipes leaving behind fractional amounts

I was playing around with mechanical filters when I noticed this with clean water.  10kg of water enters onto the pipe with 1g of water.  Instead of merging to 10,001g or swapping places, it leaves behind 1.0004g of water meaning it moved 9998.9996g over.   Is this a density calculation due to the temp changes?  Water on 1g pipe is 69C, water on 10kg pipe is 83-85 and after the swap it's 79.

If the pipe rounds up on the primary output that means you gain 0.0004g of water per second with this.  Just sayin'.