Respondents don't differentiate between their self identification as to party preference and if they actually had to register to vote with a party preference.

Since I've lived in Missouri (a fair amount of time) voters do not register by party - believe me, if we did it'd make organizing and GOTV a hell of a lot easier.

It's almost become a running gag for us when we do voter ID calls. We'll ask the party ID question and the person on the other end of the line with a voter history going back to the beginning of time (or not) will indignantly reply, "Why, I've been a registered XXXXXXXX all my life!" We never bother to reply, "You can't be, voters in Missouri don't register by party" - it'd just piss them off.