It is a glitch in the calculation of cross-matching prices. The cross-matching is a way to generate virtual prices out of the offers of the opposite type, i.e. back prices made of lay offers and vice versa. BetFair does that in certain markets, such as tennis and football.
Unfortunately the algorithm of doing that is not 100% clear, we used all the documentation we could find, but still there are some cases when these errors happen, although quite rare.
If you want to see the prices without cross-matching, you can turn off the setting "Display prices exactly as they are on the BetFair website" in "Monitoring Options".