I've just looked through every printed book I've bought over the past year and noticed a curious pattern that I'd never given much thought until now (due to currently preparing a manuscript for publication).

Why is it that some books (maybe all books?) don't indent the first paragraph directly after a chapter or subsection title but then proceed to indent all others?

Is this the standard approach? Has it always been this way, and I've simply not noticed such a minor detail? And should I mimic this?