That's been rumoured for ages. It was supposed to be Pertwee season 10, but someone got excited and commissioned S18 first. Expect S10 afterwards, and then S26. As to why they aren't doing S7-S9, the early Pertwee seasons have a lot of issues in regards to their format, color, and so on, that it would be a huge pain to restore and get up to Blu-ray quality.
As to why we aren't getting S13 first... maybe they want to keep the best sellers for later on in the range to keep its momentum. Tom's early seasons are guaranteed best sellers. They can release one of his seasons for every two other Doctors, who have proven to be far less popular. They know the show got weaker in the 80s, they're not going to go chronologically and risk killing the range before it's complete.