Thank you OutOfTime, that helps fill in some of the jigsaw pieces.
However using TFL fare finder and selecting Richmond rail (as opposed to Richmond underground) it gives the £4.50 fare. Going west via Windsor is the reasonable route for this journey if departing Richmond on a train, otherwise it would be a Zone 1 journey changing at Willesden Junction, Queens Park and Paddington. Maybe the Zone 1 routing is the recommended journey but how can the traveller know this?