rpm file name generated from spec

[Todd Rinaldo]

I'm building rpms from an srpm across multiple distributions. I ran into a problem on FC11 32bit which shows I don't fully understand how rpm filenames are generated. When the rpm file is generating, it's generating it as .i586.rpm but when I do rpm -E '%_arch' on that system, it comes back as i386.

My question: what determines the architecture part of the rpm file name when generating from spec?

Thanks,
Todd