I don't get it!

when making my chemistry, I always make the raw chemicals in teo seperate containers (100ml in each)

When mising for use, I carefully use 1:1

But when I am getting to the end of the chemistry, there's always more - much more left in one of the bottles!!

and always in the ferriammoniumcitrate bottle...

Magic/mystery? or?

I can't explain it - can any of you?