It's a good question. And the explanation, despite Hugo's comment, is just fine (except that the reference that Hugo gives would have been useful).

That explicit documentation doesn't give any explanation either. Since MONEY and DECIMAL and NUMERIC are all exact numeric types (see the Data Types topic in BoL) with decimal fractions, there should be some justification for the different behaviours but no reason is given anywhere for this difference in behaviour; why does MONEY, and exact numeric with decimal fraction, behave in this respect like the exact numerics that don't have decimal fractions instead of like the other exact numeric types with decimal fractions? I think that "for some reason" is the only reason that can be offered on the basis of BoL (although of course "because the people who invented this MONEY kludge hadn't a clue" might be some people's guess as to the reason); of course it is not necessary to offer a reason in explanations for QoTD - and in some cases (as illustrated by the potential guessed explanation suggested above) it may better not to. Why don't we have a reason? Not because the question author failed in his explanation - indeed he very accurately (albeit somewhat obliquely) pointed out that there is no documented reason - but because MS has not chosen to tell us the reason (of course there's no reason why it should, although for technical decisions which look this bizarre most technology companies do give reasons).