[CentOS] What determines default kernel after update?

I have a 32-bit machine with 8 gigs RAM and was running the PAE kernel, 
but after the last update noticed that grub was set to default to the 
non-PAE version although both were installed.  Does anyone know how boot 
default is supposed to be determined?

-- 
   Les Mikesell
    lesmikesell at gmail.com