Without going too far out on a limb with my limited legal knowledge gleaned from taking a passing interest in various incidents over the years:
They *are* different crimes but the decision as to what to charge the *offence* under lies with the prosecution and they only get one shot at it so they have to pick the one they feel they have the best chance of getting a conviction with. So if they know they'll have difficulty proving premeditation in the case of a murder they'll pursue it as second degree (heat of the moment type thing). Rape and statutory rape are different... interpretations if you will, of the same offence.
Certainly someone guilty of say.. raping a child is guilty of rape, but they're also guilty of aggravated sexual assault on a minor which will carry a heavier sentence and while it could concievably be downgraded by the judge, will still end up being a serious sentence.