Google maps is guessing at the roads taken between the trackpoints, and it's getting lots of it wrong (which is why the Ivan-miles distance is often way bigger than what Steve did in reality).
At a guess it's on bicycle routing and isn't allowing someone to cycle on the last stretch of the A64 onto the A19 and so it comes up with a wacky alternative. Adding to that, one of the dark blue line trackpoints is on the Westbound carriageway of the A64 (due to the small GPS inaccuracy) and so Google Maps routing dutifully comes up with something that includes that bit of road (hence the turn around on the outskirts of Tadcaster).
From the vertices of the dark blue trackpoint line it's clear Steve didn't cycle West along the A64 towards Tadcaster at all.
