Problem is, trying to measure and count just the theoretically countable stuff (number of vehicles, planes, ships, missiles etc) they put out is extremely hard to do.
For example, one can sort of do it for the big items. Ships for sure. And even combat aircraft to a fairly precise degree. But when one gets to less sexy systems which aren't really photographed often - like various special mission planes- then it already becomes much harder to track what China has procured.
Imagine trying to make up a list of what China got in the last 5 years.
Special mission planes?
Helicopters of various kinds?
Ballistic/Cruise missiles?
Tanks?
IFVs and APCs?
Artillery pieces?
SAMs?
all those items are so hard to track that the estimate value could easily be X or 3X for each of those items on the list.
Without those figures, it's not really any more precise than just using the defense spending figure over the last 5 years and going with that.