There have been and are a lot of different methods of delivering the appropriate amount of fuel to each cylinder, from throttle bodies (like a central carb with a distributing manifold) to "dry" manifolds that deliver air only, with an injector placed to spray at or near the intake valve. D-Jetronic fires injectors in groups of two, and therefore most of the time against a closed intake valve. Domestic injection systems can be batch fire or semi batch fire where all injectors are triggered on both or one bank of a v type engine. Ford pickup and car 5.0 litre engines are semi batch fire, except for the Mustang HO 5.0 which are sequential. K-Jetronic simply sprays fuel all the time at a particular rate, rather than in a pulse (K is for the German "Kontinuos"). Some deliver fuel in two pulses between intake valve openings.
The problem with injecting fuel on a closed intake valve is that some fuel, especially when the engine is cold, will condense on the intake port, and be moved into the cylinder on a later cycle. Compared to carbs, it is still more accurate, but less so than it can be. So sequential is best, but most complicated from a control point of view.
So it is certainly confusing, except to remember that the best systems today are in fact sequential and timed to intake valve opening points. I think in the future all systems will be, and nearly are now, sequential port type.
My head hurts now.....