Steve, it all has to do with diversity of the load. In a dwelling unit we put a lot of things in the load calculation that will not usually be going at the same time. Certainly with a house full of people on Thanksgiving with someone in the shower, washer/dryer going, every light in the house on and the stove/oven going on all burners that 310.15(B)(2) table is probably not going to be "enough" but most times it reflects the actual diversity a dwelling will have.
In the case of your garage/shop, you have usually overbuilt but the answer is just to protect your feeder with 310.16. If you are tripping that you probably underbuilt the feeder.
