The answers are already available in more detail in the relevant threads, I wish people would read them when they have questions.
Basically, HQ publishes a list of equipment of each type (weapon accessories, plate carriers, plates etc.) that are tested and cleared for purchase and use, it is then up to each regiment to purchase it’s own gear for its troops from alloted funds and keep their equipment at a certain standard, there are checks to maintain said standard in case a regiment is not keeping up. That’s why there can be multiple different types of the same thing in service, one regiment purchases one type, another goes with another, as long as they are from the approved list, it’s fine. Yes, a single standard is more ideal, but that is in a way more expensive and difficult to accomplish. (And keep in mind this might not be exactly how it works, someone else can probably explain it better or add to it/correct it, but that’s the basic idea behind it from my knowledge).