Permission Based Emails? Are you sure?

Yesterday I wrote about the ReturnPath study showing 21% of permission based email does not make it to the inbox. There are a number of reasons I can think of for this result, but I think one of the major ones is that not all the mail they are monitoring is permission based. I have no doubt that all of the RP customers say that the mail they’re sending is permission based, I also have no doubt that not all of the mail is.
Everyone who sends mail sends permission based email. Really! Just ask them!
In 10 years of professionally working with senders I have yet to find a marketer that says anything other than all their email is permission based. Every email marketer, from those who buy email addresses to those who do fully confirmed verified opt-in with a cherry on top will claim all their email is permission based. And some of the mailers I’ve worked with in the past have been listed on ROKSO. None of these mailers will ever admit that they are not sending permission based email.
Going back to ReturnPath’s data we don’t really know what permission based email means in this context and so we don’t know if the mail is legitimately or illegitimately blocked. My guess is that some significant percentage of the 20% of email to the probe accounts that doesn’t make it to the inbox is missing because the sender does not have clear recipient permission.
When even spammers describe their email as permission based email marketing, what value does the term have?