It seems reasonable to me that since she is visiting you, and that she is a dependant, then her visa should have the same duration as yours.
Of course, reason is nothing to go on in these matters, nor do rules, regs or anything else with any kind of regularity. Only when it's convenient for the guy 'making decisions' (although I use that expression lightly).
We got our daughter's visa changed from tourist visa to visiting relatives when we got here. My embassy tells me with the recent rule change and more stringent enforcement, this is no longer possible. (tourist visa to any other, business visa to any other)
However, my FAO says no problem, and so does my wife, who actually gets our visas for us, not the uni, although she uses the uni's paperwork, of course. Daughter's visa didn't use any paperwork from my employer at all this time, although up until then we always have. Mrs N says it's simpler without the uni's paperwork for Little Miss N, although, of course, necessary for mine - but she says she is working on that too. Actually, she started getting my visas and daughter's visas processed before the regs came in about having to front up in person, simply because our FAO at the time was a lazy incompetent baijiu swilling butthead - although a nice guy, and treated me well, mostly by leaving me the hell alone.
In the process, she has learned a lot about the rules, but I can't understand it half the time. Her English is great, but my understanding and patience with trying to comprehend the intricate details of Chinese bureaucracy is virtually nil. I don't even like understanding my own, and I worked for the DoD for several years as a bureaucrat.