Why did Comey mark some of his memos unclassified?

In other words, after writing a series of SECRET or CONFIDENTIAL memos detailing his interactions with the president, all of a sudden Comey starts treating his conversations with Trump as UNCLASSIFIED. Which brings us back to the question: Why?

It can’t be because the material discussed was uniquely unrelated to matters of national security or ongoing FBI investigations. After all, it is the February 14 meeting at which Trump discussed the dismissed National Security Advisor Michael Flynn, the meeting at which Trump famously said (according to Comey) “Flynn is a good guy and has been through a lot … I hope you can see your way clear to letting this go, to letting Flynn go. He is a good guy. I hope you can let this go.” Thus, the conversation involved discussion of what can be assumed to have been, then, an ongoing investigation, the very sort of thing that would be at the very least CONFIDENTIAL if not SECRET. And yet in writing the memo, for the first time Comey specifically constrains what he says and how he says it so that he can give it an UNCLASSIFIED designation. Was this because Comey was already strategizing how to leak the information without putting himself at risk of being found to have released classified information?