You're starting them in order and you're ending (retiring) them in order, but you're not necessarily ending one instruction before you're starting the next one.

You're starting them in order and you're ending (retiring) them in order, but you're not necessarily ending one instruction before you're starting the next one. For instance, in a very simple pipeline, you can start decoding the next instruction before you've completed the previous one, so you can do some work in parallel.