> For the life of me I don't understand why you'd ever want to do an atomic operation that's not naturally aligned, let alone one split across cache lines....

I assume they force packed their structure and it's poorly aligned, but x86 doesn't fault on unaligned access and Windows doesn't detect and punish split locks, so while you probably would get better performance with proper alignment, it might not be a meaningful improvement on the majority of the machines running the program.