Working out head to head odds given tournament odds

[Jimaldo]

Hi guys. I'm trying to start work on a system that looks for value in head to head odds by somebody's tournament odds. Looking at the darts, (and adjusting to presume a 100% book), Taylor has a 28.7% (3.4) chance of winning the Worlds, whereas Mark Webster has a 0.23% chance (420.0). So playing each other head to head I would think that Taylor would have a 0.287/(0.287+0.23) = 99.2% chance of winning that match - or odds of 1.008. However his actual odds are 1.08 (92.6% chance). Calculating the head to heads as above always underestimates the underdog and overestimates the favourite. What can I do to improve this predicting? Thanks for taking time to read and any suggestions welcome :)