an ammo box can hold less ammo when in your hands compared to when it is on the ground (on the ground 10 slots and in your hands 5 slots of carrying capacity).
[if you completely fill the box while it is on the ground it can carry the full amount it is supposed to be able to hold. you can't access the 2nd row of slots while it is in your hands tho'!]
compare the amount of slots of an ammo box in your vicinity and in your hands.
The only way I can add ammo after the 5 slots in hand is to put it on the ground to see the additional available slots. When on a second floor placing it on the ground sometimes makes it fall through the floor. It looks like it's gone but showed up when I went down stairs.
Also Emptying ammo out of a box and adding it to the ammo box while in inventory will place it in a new slot and doesn't combine it. you can't combine the lower unseen slot ammo until you place it on the ground.
from what i have seen the second row is still accessible ... kinda
you can still fill all slots, but when it is in your hand you only have direct access to the first 5 slots, you need to remove ammo to inventory so the ammo in the hidden slots move to the visible ones.
not a good way to do it
guess the report needs to be the poor interface system when equipped in hand