Only charged for Zones 1 and 2 for a journey via Zones 3 and 4

I frequently travel to London to visit my parents in Putney, and until the rise of the Omicron variant of Covid my usual route after arrival at King’s Cross was the Victoria Line to Vauxhall, then National Rail to Putney.

However, on my last two visits I decided, as a precaution against Covid, to avoid using the Tube and take a route (albeit a somewhat circuitous route) that only passes through, and involves changing at, surface stations rather than tunnel stations.

When I went to stay with my parents for Christmas, I walked from King’s Cross to Euston and then took the Watford DC lines train to Willesden Junction, the North London Line to Richmond, and South Western Railway train to Putney using the pink card readers at both Willesden Junction and Richmond.

On my most recent visit I did the same except that I took the West London Line from Willesden Junction to Clapham Junction then SWR train to Putney, again using the pink readers at Willesden Jn and on the London Overground platforms at Clapham Jn.

I was surprised to find that I was only charged for a Zones 1-2 National Rail journey (£2.70) both times even though Richmond is in Zone 4. My understanding of it has always been that you are charged for all the zones through which you travel, not just for the zones where your journey starts and finishes, especially as in the FAQs section you say that a Zones 2-5 fare would be charged for a journey from Streatham Common (Zone 3) to Twickenham (Zone 5) via Clapham Jn (Zone 2).

Obviously a Zone 1-2 fare would be correct for a Euston-Willesden Jn-Clapham Jn-Putney journey as both Willesden Jn and Putney are on the Zone 2/3 boundary.

I’m not complaining if I get charged less than I thought I would, but I am curious as to why this is.