Not all distributions will run the latest kernel as they will want to run the same version to keep compatibility since the distribution version has been released ... particularly common in the enterprise arena.
RHEL5 for instance runs 2.6.18 (*) with RHEL 5.4 having 2.6.18.164 and 5.5 having 2.6.18.194. Fixes, particularly security ones, will frequently be back ported to the kernel they are using by the distribution maintainers but the same basic version is kept to keep compatibility with enterprise applications.
(*) exact numbers are off the top of my head and might be wrong ... but even if they are then the point stays the same.