Orange 021 is just another odd name Pantone came up with. Keep in mind that spot colors are actually hand mixed at the print vendor's shop. Once mixed, the print vendor might keep the ink in stock for future use. Each spot color has a formula associated with it and is measured in ounces or pounds ( not sure which ). There are ink companies that will batch colors for a print vendor. I have an older Process Color Converter wheel from The Color Wheel Company. It has all of the Pantone 1000 CMYK equivalents on it. I looked up Pantone Orange 021 and I get 0%C, 51%M, 87%Y, and 0%K. I wonder if you get the same numbers from your Plus Series book?
Spot color is more accurate because CMYK can shift on press. If just one of the four process colors is off a few percentages, then the color will drift. You'd have to have had to actually run a press to understand what I am saying.
I will write more later.