Who knows? Probably to make themselves more attractive to the other. I mean you wouldn't even try with someone who wants completely different things from the relationship would you? Like if you met a guy and he told you out right that he's not looking for anything long term would you pursue him if you are looking for long term?
There are of course exceptions to this and this is only my opinion on why someone might lie about what they want from a relationship. Honestly I never understood why you would put yourself through any of it. The lying or the hurt when things just don't turn out the way you want because you or the other person lied about what they wanted and it is inevitable really.
I've seen lots of friends change themselves for potential partners and it always ends in heart ache because it's false.

They really have no idea what they want

Afraid of being judged, being rejected, not appearing to be "normal", don't know what (s)he really wants...there are many possible reasons why!