Unit's don't scale properly.

[mother-east]

Hello. Im trying to create real world models and the unit sizes are all wack, in fact they are double even though they say its the correct size. Before any modelling i change the preferences to project scale 1 mm, and i change it to units to mm too.

I create a circle thats 66mm wide and then when i use the measure tool its always double? i have no idea why this is happening but its very annoying.

So what ive had to do is scale everything down by 50% for it to even look correct. You might ask why not just make it half the size, like 33mm instead? i tried that however the scale is still off until i scale it.

Any ideas would be great.