Unitful @refunit converts to other length units

General Usage
,
1

I define a pixel as a reference unit as follows

@refunit px "px" Pixel Unitful.𝐋 true

However, the following happens despite not defining its size,

 uconvert(u"mm", 1u"px")
1000 mm

Is this expected?

2

I don’t see that I redefined px anywhere in the code. I don’t know how it makes the association to other SI units.