When to use "explain why" versus "explain why/why not"

I had a question regarding… another question I encountered.

The question is phrased like so:

Is water a gas? Explain why.

That’s not the actual question, but the ideas by the supposition are the same. To elaborate, water is generally thought of in its liquid form, but water can be gaseous, but, it is usually referred to as water vapor if it is gaseous.

Essentially, the way I read that question is that it is asserting that water is a gas, and I should be explaining why.

But, if the question was written like this:

Is water a gas? Explain why/why not.

I’d probably be more inclined to read the initial question as less of an assertion, and inevitably that it is more of a prompt that water isn’t a gas and I should be explaining why it isn’t.

Are both my interpretations correct? Or is the existence of the "/why not" not relevant to how I should interpret the question?