TDS rising?

[HarleyC]

Hi,
I run my water through a Aquarium Pharmaceuticals filter - http://www.marinedepot.com/Aquarium_...FIRODI-vi.html

I'm in Vancouver, so even before it goes in, the TDS reads no more than 10.
After the filter, I get 0 as you would expect.
However, if I then check it a couple days later in my top up bin, it reads 10. Why would this be?
I don't think it's the bin, as I can check it in the bin when it is filled and it reads 0.
Thanks