I would assume that this is all the more so for airframes, but it may also apply to many other items.

Remember that plane boxes are quite large and fragile, so shipping these is a big issue.

It behooves the vendor to ship out only large lots, that can be split at the desitination.

I know that some of the Chinese factories will receive an order for a specific lot of planes, tool up for them, and once the order is filled, they retool for other items until they receive another order large enough to warrant retooling again.

That is why parts can be hard to find at times, and when you DO NOT seem to need parts, they are plentiful for a particular plane.