FastEagle is right, 60 psi is not a standard tire sidewall "max psi" number. 65 psi is the right number for D rated tires. So, unless you have a pretty old trailer, it sounds like the manufacturer was smart enough to put 60 psi on the sticker based on tire loading at the trailer's GVWR rather than just restating the sidewall max psi.
This is no different that what vehicle manufacturers do, all of my truck tires are E rated, i.e. 80 psi, but the sticker specifies 55 psi for the front tires based on front GAWR rather than the sidewall max number. And while the rear tires are supposed to be inflated to 80 psi, many of us will run significantly less than 80 psi when the truck is not loaded.
This is a round-about way to say, weigh the loaded trailer and use a load inflation chart to determine the correct psi.
Larry Day, Texas Baptist Men volunteer
'13 Silverado LT 3500HD D/A CCSB 2wd, custom RKI bed
'11 Crossroads Cruiser 32MK loaded