As someone who once wasted a year of their life working for a publisher there is (sort of) a rationale for some sizes. Manuals for example are usually standardised at US Letter size because they are frequently referred to.
Essentially though you are probably irritated with what's known as "trade size" - the larger paperbacks that won't fit on shelves. These are essentially the paperback version of the hardcover - same pagination and layout. That's what generally goes out for review purposes and thee days usually comes out at the same time as the hardcover.
It's also significantly more profitable than the normal paperback and if a book looks to be selling well in "trade" then the standard size may be "delayed" a while to take advantage of those buyers who purchase as soon as it comes out.
