Hiya, I lurk here quite a bit and I'm waiting on a renewal but I thought I'd explain roughly how the DWP works for stuff like this (I used to work for them in a different department)
There's more than one office in the UK for DLA and each of those offices will have several teams of people who work on different parts of a DLA claim. So there will be admin who put in all of your details and make sure it's all correct and make sure you're eligible, there will be another team who focuses on MRs etc., and then there are decision makers/case managers.
As an individual decision maker you will have your own personal set of claims you need to work through, and obviously some will be quicker than others regardless of scan date because they're easier decisions. The ones that take longer will be because they are having to get in contact etc schools or other relevant professionals and they might take ages to get back to you, so you would work on the other cases in your caseload while you wait for the information you need to arrive. Those claims don't go back into the system for other people to pick up as far as I'm aware (even if you're on annual leave) and your manager would be expecting you to get through a certain number of claims a day so you will inevitably finish one that has a later scan date while waiting for further information for others.
They desperately need more staff but due to funding cuts under the previous government it's difficult to do as their budgets still aren't as high as they need to be.
I just thought I'd give you a bit of insight into how it works, at least from the perspective of someone who worked for DWP in a different department as it is extremely frustrating and completely understandable why you're upset about others being awarded before your little one is.