I'd bet that instrument functionality code piggybacks on some of the code from weapon functionality, and they must've changed delay timer between weapon swap and key press. And since the original code from instruments was probably created in the early gw2 development, it's possible that the person/persons who coded them no longer work at arenanet. And since programmers have a tendency to make messy code (which sometimes they do on purpose to tie themselves to that section of code so that if a company fires them, they essentially wouldn't be able to easily adjust that code without the original programmer), it's possible they don't know a way to detach instrument functionality from weapon functionality.