Just in case there’s someone who has an interest in such things but was afraid to ask ;), here’s a recent blog post that covers how and why security software uses different drivers on 32-bit Windows to 64-bit Windows: http://malwaretips.com/threads/why-antivirus-software-may-use-kernel-mode-drivers.44822/
I know it’s specifically about antivirus (not to mention somewhat geeky) but it directly concerns, and is linked to, other types of Security and Privacy software as well.