Paulo,
When the guy first paints 21 on a runway the actual runway QDM can be anywhere between 206 and 214 degrees. After several years of annual change of magnetic variation the runway QDM could be well outside these limits.
I think what Bob is referring to is that scenario where somone lines up on Rwy 21, sees the Heading Indicator on 202 degrees and realigns it to 210 degrees. The compass meanwhile is also on 202 degrees.
Now where's the logic ?