Pipes and fluid buffer flow rates

I had a refinery making alumina+silica at 250% and slooped, so it uses 450 water. I had a downstream refinery making scrap at 250% and slooped, so it makes 400 water. I had other downstream scrap refineries, so all the alumina would have somewhere to go.

I've been having water recycling problems, even with a VIP junction. Edit: Since it's possible my build of the VIP junction is part of the weird flow problem, I've posted screenshots of the VIP junction for the refinery that I was using to test the buffer stuff. The fresh water was not hooked up while I was playing with this buffer stuff.

Anyway, not the point of this post.

I wanted to see if putting a buffer for water between the refineries would help. One thing that surprised me is I would see the flow rate go between -150 and +250 in the buffer, seemingly 2/3 of the time >0. At one point it had 1200 in it. I went and messed with my rocket fuel base, which is out of rendering distance for maybe 20 minutes. When I came back the buffer was empty.

I expected the buffer to be empty, but I don't understand how it ever got as full as it did. I attached the buffer when the first refinery was out of water and the second refinery was out of alumina. There was probably leftovers in the water pipes that drained back at first, but I quickly put a pump on the output side (buffer below the refineries to play with it, not final spot) and a valve on the input side. The pipe water would maybe be what 100 or so (refineries are front to back with a foundation between them).

I would see the wastewater pipe have a flow rate of 300-400 a lot of the time and frequently after the pump I'd see 600.

Anybody know what's going on?