i've got a 16" summer season and agree with the others yours in defo a 16".
Looking at on-ones website, they seem to measure the vertical height of the seat tube (dimension E in link below), not the centre to top, which might explain it.
http://www.on-one.co.uk/i/q/FROO456/on-one-456-frame
I was about to say, get the size up and put a slightly shorter stem on it to get the reach the same, but just noticed on on-ones website that both thw 16" and 18"s have 105mm headtubes, so that wouldn't raise the front end.
So, as someone above said, if you don't like all the spacers only option is a higher rise stem.