She just came out, and still in beta, so, this is pretty much expected.
I'm not a native speaker too but I have no problems with her understanding me. Human factor may have taken part in this one.
Just came out? She's out there ever since MS released windows 8.1, and leave alone other countries, US still has BETA. Can we even compare the progress that google now is making in terms to voice assistant? I don't want Cortana to say a joke or some weird answer back to me but do what it is supposed to, which google now does pretty good. I use alpha version and its been ages that there has been no update and it is just like some damn reminder app except that it sometimes works on voice.