AFAIK, "Missing" here means the email did not arrive in a user-accessible mailbox, neither Inbox nor Spam (hence disposition could not be determined).
Inbox=able to verify it reached the actual inbox
Spam=spam box, junk folder, or known spam filter block
Missing=did not see in above two, which usually means the Server or Link scanner blocked it well before it got to Spam box.
Generally, you should hope to be at 90% inbox rate if you are running a clean list with a good IP reputation. Generally, B2B emails have been falling below that more recently, prob due to the mass cold email craze.