Well I can think of 6 reasons....
1. There are only 5, but some have been resprayed during their life and old and new images exist on the 'net
2. There are only 5, but some 'ordinary' XJ220 owners try to pass there's off as TWRs
3. There are only 5, but some photographers don't know the difference and have labelled their images as TWRs
4. There are only 5, but some photographers know the difference but have labelled their images as TWRs to make them sell better or them feel important
5. There are only 5, but some XJ220 owners have bought new engines and "upgraded" theirs
6. There were more than 5 built
With no other evidence it could be any of them, but gut-feel says a bit of 1 and 2