This is a bit of a debate about of this painful condition in children should be called Severs disease or calcaneal apophysitis. They are both the same thing. Severs disease is the more common traditional name and Calcaneal apophysitis is the less common, but technically correct name.
It is not a disease and the trend is to no longer name conditions after people, so it is correct that the name Severs disease should be dropped and the replaced with the name calcaneal apophysitis. But, when it comes to the search engines like Google, the vast majority of searchers are fro the phrase, Severs disease and very few people searching for the name calcaneal apophysitis.
This issues was discussed in this episode of PodChatLive: