Mr bob, there is some logic to this...
...compasses can be horribly wrong. Unless some joker has repainted the numbers, the runway is usually right.
This might sound a bit theoretical, but in the type I fly, the compass is ******ed if, for example, you have pitot heat on.
Of course you shouldn't have it on when setting DI on initial checks, but who knows. I never realised this 'throw' until the CFI mentioned it to me one day. 20 degrees 'ping!' on pushing that switch.