The boiler tube thickness spec that blew my mind last Tuesday

I've been fitting boiler tubes for about 6 years now and always just followed the prints without thinking much about where the numbers come from. Last Tuesday I was waiting on a crane at the Long Beach yard and flipped through an old ASME code book that was sitting in the gang box. Turns out the minimum wall thickness for our 2-inch tubes is based on a formula that accounts for corrosion allowance plus a 12.5% tolerance for manufacturing. I always assumed the thickness was just rounded up from pressure calculations, never knew there was a built-in fudge factor for wear. It made me check the actual caliper readings on the last retube we did at the refinery, and sure enough most of the used tubes we pulled were right at that corrosion line. Has anyone else ever dug into the actual code math behind a spec and found it looser or tighter than you expected?