A bug or a feature? Sometimes it’s hard to tell for sure.
Web services are a fantastic tool in NAV, however, they do not always behave exactly as you would expect them to. There is one particularly annoying behavior, which just after you get used to it starts getting even stranger.
So, setting any numeric value during the Create method call on any page web service will have no effect, and the only way to set a value in a numeric field would be through a subsequent call to Update method.