When connecting between different tube lines, much of the time users remain inside the ticketed area of the station, so only tap their Oyster/contactless card when they pass through the entrance/exit barriers at the very start and end of the journey. (Though there are a few stations where there is not a direct tunnel between lines, and travellers are required to connect via a separate street-level entrance, where they would tap their card mid-journey.)
In cases where there are multiple routes to complete a trip (eg remaining on one line vs making multiple connections for a faster journey) it was therefore previously not possible to determine what percentage of people chose which route.
I'm sure the data from that is much more fine grained.
Update: Ah got it. With Oyster they can only register the entry and exit points and not the route taken.