I'm surprised no-one has noted the obvious situation in which the value of PI might change. It's not π the natural constant the manual is talking about, it's PI the variable in a program, to be run on a machine constrained by finite storage and precision.
Depending on your decade, your machine, and your purpose, you might start out with CONST PI = 3.1416 then gradually refine it as you get more storage or need more precision. A one-line change to CONST PI = 3.14159 and beyond makes perfect sense.