Regions matter because in some areas of the country the weather is mainly cooler than others and so for the most part all Southern areas of the country may require a little less insulation than those in the northern part of the country.
If you look at the map that the department of energy put out (see below) it has eight different zones across the country and depending on what zone you're in will determine what is recommended for attic insulation, wall insulation, and floor insulation. For example in zones 1, 2 and 3, which are located in the southern areas of the country like Southern California, Southern Texas and Southern Florida, you can get away with a little bit less insulation in the Attic like an R30. The rest of the country is recommended that you use a minimum of R38 up to an R64 insulating the Attic. Likewise for flooring in zones 1, 2 and 3 you can get away with a little bit less insulation like an R25.
Where the DOE is recommending higher insulated value than the rest of the country is where it gets a little bit cooler. So depending where you live in the country will determine what R-value you need for your attic and your ceiling. Take a look at the insulation map below determine what R-value you need for your Zone.