This is currently intended - due to some issues in Steam's x64 support and delivery mechanism we decided to press ahead with a 32-bit only release rather than delay to fix the x64 issues. It's no major concern though since the features are identical. We're working on bringing x64 support, as well as beta updates, to Steam in the near future.
I also have the Steam version and 64bit windows. Can I download the 64bit version and activate it with my licence? Also I'm missing the email containing my licence and my link to download the bonus sprites etc