The domestic dog is thought to be the most varied mammal on the planet. The Kennel Club in the United Kingdom recognizes just over 200 different breeds of dog. To provide an accurate ranking of a breed's propensity to go AWOL, one would need to ensure that a) enough owners of each breed were covered by the survey and b) the results for each owner group were isolated before ranking.
For an ideal methodology template, see the Consumer Reports annual survey of car ownership satisfaction. To rank the satisfaction levels of around 350 different models, Consumer Reports surveys 350,000 car owners (around 1,000 owners for each model covered) then compares satisfaction levels across the groups of model owner.
The Blue Cross survey simply couldn't have surveyed enough dog owners to ensure all breeds were fairly represented. It also seems unlikely that the results for each owner group were isolated before ranking. Indeed, ten of the dogs in Blue Cross' list of the top 15 runaway dogs also feature in The Kennel Club's list of the top 15 registered dogs with the Retriever and Cocker Spaniel sharing the same top two spots in both rankings.
All that the Blue Cross results provides is a list of the most popular dogs and confirmation that dogs runaway sometimes.