@rook : you should run a diff on the two kernels, after all those years (17 since the split) of divergence there isn't that much that is still common, however there is still cross-breeding between projects and a really good idea will spread all around.
OpenBSD got way more in the way of security architecture, as in compiler support to prevent buffer overflows, address randomization all around so nothing is predictable by an attacker, strict memory protection so writable memory is not executable, that kind of stuff making any bug that would have passed its developers prying eyes way harder if not impossible to exploit. It also makes running insecure software a bit safer.
OpenBSD was also first to introduce many advances, especially in the field of practical cryptography, e.g. stronger password hashing (bcrypt), swap encryption... Stuff that won't affect any exploit count (which doesn't mean much as a security metric, if one didn't find a bug maybe he merely didn't look hard enough) but does a lot for practical security. Also in the practical security part you could see the widespread privilege separation, widespread chrooting as things that should bad stuff happen, will make it a lot less bad...
Disclaimer: I used to be an OpenBSD committer, but it was a long time ago.