A

Because they're vicious, cruel people?

I'm just kidding. I'm from the US and have heard 'a' pronunced a half-dozen different ways. Like every other language, there are different accents.

Maybe they were trying to emphaisize that it was only one pen?  "It was *a* pen, not two!"  Many speakers use /eI/ before vowels rather than /@/, but y'know sometimes odd things happen, and they might use /eI/ before consonants as well.  When you sing, you almost always use /@/ before consonants, and /eI/ before vowels.