vertexPositions should be a `ConstantProperty` not a `ConstantPositoinProperty`.  It's an array, not a single position.  `Property` is an interface, that means all properties have a function called `getValue` that takes a time.  However, `ConstantProperty` doesn't care if you pass in a time or not; but normally the people calling `getValue` have no idea what the type of property is.  So the counter-intuitiveness you descrive isn't encountered by anyone simply using the API.