Doesn’t this just mean you’re using a different part of the cassette? or am I missing something?
		
		
	 
A few interesting replies to this question.
Today I probably had to use a wider range of gears, but maybe that finer granularity just gave me a chance to hold my power better.  With less changes, I think your cadence probably plays a more definite role in holding power? 
On this ride, the resistance seemed more constant, supported by the gear changes, and it felt that I was holding power more by maintaining pressure (grinding as 
@Whorty said) rather than through spinning.
Of course, I'm much stronger than I was a few months ago, so that has to be a factor as well.  I'm wondering whether stronger legs plays a factor in being able to roll a bigger gear, push against a higher resistance.  It must have some effect?
Whatever it was, I got up Leith Hill 51 seconds faster and I was purposely not going in to the red.
Its a one-off, so too early to draw any conclusion until I've tried other courses, but worth playing around with this a bit more I think.