Plot spectrum null test in Audacity

I import a wav track into audacity and then again. So they both should be bit perfect. I invert one and then i select some region of both tracks and then click plot spectrum. Instead of seeing nothing i am getting a bump at about 22khz. Nothing else anywhere else.

Can someone explain why this is?

Surely if i import the same track twice, they would be equal and if i invert one then the sum should not contain anything?

Dither noise ? … Dither - Audacity Manual

thankyou. Yes switching dither off fixes the problem.