ARB bushes size

I measured the arb on the front of my car and it was 20mm.

I put my reg into micks garage and it has came up 19mm.

Question is does a bushes be 1mm in difference or have I measured the arb wrong.

I got 20mm bushes of dave so don't want to fit them if there wrong size.

Dave would know but unless you measured them with a calipers you won't be accurate