reduced ammo loadout of 2S3 SPHs?

[Shan]

I am using self-propelled artillery in some scenarios I am working on parallel, and this is how I found that there are some discrepancies of the ammo loadout of these systems, depending on which country uses them. So I looked up the 2S3 in the limited literature I have on this subject (Tanks of the World, Edition 8, ISBN 3-7637-5984-0, as well as the book on the East German army I mentioned in the respective OOB thread), and I found out that the basic 2S3 can carry 40 grenades, whereas the 2S3M and -M1 carry 46.

Now, it's clear that the type of ammo carried varies from country to country (availability of cluster ammo, doctrine (use in direct fire role - HEAT), etc), and I do not have any source on this, but I believe that at least the ammo stowage should be used to the maximum. In the current OOBs, however, standard 2S3s carry only 33 rounds in various OOBs I checked (30HE + 3 HEAT for Russia), whereas 2S3M and M1 in Russia and other countries carry a total of 40 rounds.

So - does the info I got on this most likely only refer to a theoretical maximum loadout which is being reduced for practical purposes (lack of space for crew?), or why would anyone send a vehicle into battle without using its full ammo loadout?