I should clarify.

My assumption is that any instances of British spelling in the document(s) were accidental.

The petition, otoh, implies that Carney's office has adopted a policy of using British spelling.

The 'new policy' explanation is more surprising than mine.

It makes a difference.

If the conventional explanation is the right one, then this fuss is over a few minor spelling mistakes, as opposed to Carney exercising poor judgment.