From: Washington, D.C.
NATO has a set of rules for assigning a standardized code name to a weapons system. Changes in naming can reflect updated assessments of the weapon's capabilities, or updates in the rules. It could also be that there's multiple versions of the same weapons system with different capabilities reflected in their name. I also have it on good authority that occasionally, creative thinking intelligence analysts give new things their own names in the absence of any official name, which make sense to them. E.g. "That missile looks like an anteater!" and it becomes an "ANTEATER." That name then sticks for a while, in spite of it not following the rules strictly. Later, as assessments are updated, they might be assigned an "official" name, which then results in a naming inconsistency in the literature.
Don't forget the option of WSSIC not assigning a name/designator until well after IOC, then classifying it. The public literature on WSSIC IDs/names on pretty terrible for a lot of post-Cold War systems, leading to a lot of the above inconsistency.