This is an answer in general terms rather than specifically about this IC.
It's poor form for the manufacturer not to explicitly state the functional differences between the versions of the IC in different packages. But, as Rumsfeld would say, you design with the documentation you have, not the documentation you might wish to have. That is unless you are on good terms with a Field Application Engineer at the manufacturer, in which case you can ask her to clarify.
In the absence of other info, it's likely that all package variants use the same die, and those with "missing" pins simply leave some wire-bond pads on the die unconnected, so the IC will behave as though you had chosen one of the variants with all pins but had left some pins open-circuit. However, that isn't the only possibility. For instance, if there's some configuration input pin that the user is supposed to strap high or low and not leave floating, the IC manufacturer might use an internal bond wire to connect it to VCC or GND in a low-pin-count package variant and thereby not have to have a package pin for it, at the expense of configuration flexibility.