Mistake, or Blunder?

As Krafty intimated in his Attacking and Defending post, he and I have nerding out and playing a lot of chess online in recent months. I’ve also been playing against his father, who employs a two-pronged approach of (1) being very good and (2) taking FOREVER between moves, such that its very hard to feel that you’re ever making any progress against him. Some easier pickins finally came along in the person of my friend Ryan, who mentioned having played against various math experts (or something like this), but  turned out to be not very good and relatively easy to subdue.

Any you finish a match on chess.com, a button appears that invites you to click it to receive ‘Computer analysis’… but every time I’ve done this, I’ve simply gotten a message asking me to wait a very long time, after which nothing happens. For whatever reason, when Ryan clicked the after our match, he actually got a report that included the following taxonomically-curious information:

• Inaccuracies: 6 = 31.6% of moves
• Mistakes: 3 = 15.8% of moves
• Blunders: 4 = 21.1% of moves

Now, inaccuracies I suppose just refers to any move you make that’s different (and, therefore, less accurate) than the one the computer would have made. Maybe. But I’m dying to know the difference between mistakes and blunders. Does the percentage of blunders include the number of mistakes, or are they counted separately? (If separately, that would mean that the computer is essentially telling Ryan that a full two-thirds of his moves were bad). Is there a level worse than blunders? Oafish calamities?

The whole thing reminds me of the mysterious classifications that used to lurk at the bottom of the IQ scale (before they cleaned up the terminology to use less pejorative terms):

This is weird, I think, because most people would probably think of these terms as synonyms, not as a hierarchy of mental capability.

In any case, I think the chess analyses would be much better if assigned some final judgement like this at the end.