Postcodes are determined by the routes used to deliver post. The boundary shown in the map will probably have been set before the Inn was assigned a post code (possibly due to not existing then). However, if you think about the route you would have to take to deliver post to it, it logically belongs in N17, rather than E17, which would, I suppose, require the postal route to divert across the water simply to fit in with an old boundary. Practicality trumps pedantry.

I think I explained it wrong. Normally Streetmap is pedantic about postcode boundaries; what I would have expected was for the red line to have diverted around the pub.

Perhaps Streetmap bought its postcode boundary data before the pub was built, and the pedantry was on the part of the third party that compiled that data. They then wouldn't see any point in updating it for what will be a few minor exceptions.
Or... the pedantic postcode person has left the company

Dont think so — the pub's been there for quite a while, according to the internet. The Shady Old Lady says it's been there since the 1700s.

Ha! This page says it was the site of a cops and robbers tram chase in 1909! Brilliant!

Knowing the area (and the pub) I'd suggest that it was sensible for them to put the pub in the Tottenham postal district rather than Walthamstow. They've been flexible with postcodes before, I think...

I think I explained it wrong — see my reply to Susannah.

While not an answer to your question, we have a rule when teaching map-reading and navigation.

"In the case of a discrepancy between reality and the map, reality is correct."

Sam

PS.