In which case they're erroniously terming themselves Atheists.
Strong Atheism (which is what most people think of when they say "Atheism) is the rejection of Theism. You're not just living without Theism, you are actively rejecting it from your life.
Weak Atheists are atheists. Strong Atheists are atheists. (Also may be known as Implicit and Explicit Atheists, respectively, give or take some fine distinctions that I won't bother going into.) Whatever kind of atheist you are, you're not being erroneous in calling yourself an atheist. The problem is in the 'which is what most people think of when they say "Atheism"' bit. Whether the problem here is with "most people" having it wrong[1], that the Weak Atheist should not be using a term that is obviously not understood properly or a mixture of the two is very arguable. I know, I've seen it being argued (and participated).
[1] c.f. "Decimate". Now,
I know that this derives from 1 in 10 men in a Roman military unit being killed (by their colleagues, according to a lottery system), but most people think of it as akin to "annihiliation" or (with more rationality) a reduction
to 1 in 10 of the original, and then go on to use it in some form of hyperbole regardless. So, should I use "Decimate" as I think it should be used, 'give in' to the incorrect meaning that has arisen and accept that not evolution of the English language happens[2][3], or save up my distaste for the incorrect meaning until that I find myself in the situation that I can sneakily use it in a manner that I can willfully allow to be misinterpreted to my own ends while remaining totally truthful?
[2] Aside from the fact that it's an adopted word, anyway.
[3] I may not fight to the death over "your"/"you're" and the abomination which is "could of", but I'd risk a Chinese Burn or two in pointing out peoples errors in that regard. Also errors/redundancies like "PIN Number" and "ATM Machine".