Different tyre size from one brand to another?

[nielshm]

My Corolla uses same size 205/55R16 as summer-and winter tyres. Summer tyres are Continental, Winter tyres are Bridgestone. 

My speedo reads very accurate (3%) with summer tyres, and they are down to 4 mm. tread. 

With winter tyres things are different. Now the speedo reads within 6% error, and the tread is 6 mm. 

Why is that? Same size by default, but very different in accuracy. As the tread is worm, speedo error should increase. So my winter tyres could end up with 7-8% error? 

[Roy124]

The cold rolling radius of the winter tyres is greater and would give the results you calculated.  However the winter tyres are softer compound and may have greater deflection on contact. You are only looking at a static difference of 2 mm so it would not need much to change the effective radius. 

Just speculating.