Sorry, I would have liked to leave this as a comment instead, but my rep isn't high enough yet.
Personally, I think it's just a style issue. No actual grammar rules have been broken. Having "an" would make things more clear, but they are not unclear without the "an."
Unfortunately, I couldn't find a grammar page regarding this issue exactly, but if it helps, you're not alone in asking questions about dropping indefinite articles:
And I totally agree with Damkerng's comment in the original post. I had found it interesting as well that the original speaker of the sentence had omitted the article in the first part and not the second.