meaning - Is there a semantic difference between "manipulable" and "manipulatable"?

In all the sources I can find, the terms "manipulable" and "manipulatable" are both defined as some form of "able to be manipulated". But depending on the source, one word seems to be related to mechanical motion whereas the other is related to the act of persuasion. None of the sources I can find list the two words as actually equivalent to each other, so I can only assume the mechanical/persuasive split is intentional.

Is there a common understanding or historically-supported difference between the semantics of the two words, or should a dictionary list them as alternate forms of each other?

Answer

You neglect that "manipulate" already has a suffix. The word "manipulatable" applies a suffix to an already-suffixed root.

Consider the difference between the known meaning of the word "manipulate" and the supposed meaning of its root. "Manipulate" describes a process of controlling something. That process part is very important--it's what the affix "-ate" brings. The root, then, means at its essence the inherent property of being controllable. We don't have a word that's based on this root for that, but that's important to understand for appreciating the difference here.

"Manipulatable", therefore, means that a thing can be worked especially because the manipulate root is already affixed to describe the act of controlling something. When you say "manipulatable", you're describing something that can have that process applied to it. Thus the relevance to mechanical processes. This definition is used much less often than "manipulable" is is likely far newer; the distinction is likely maintained overtly and intentionally, to provide a more specific word than "manipulable" brings. Not all sources recognize "manipulatable" as a word.

The word "manipulable", on the other hand, skips the "-ate" affix and goes a step backwards to its root. It forgoes description of a process or action. Instead, it's describing something as "capable of being controlled", which makes sense in many more contexts besides those that can be described as "manipulatable".

So there you go. All things that are "manipulatable" are also "manipulable", but not all things that are "manipulable" are "manipulatable".