Hi Dracula,
You have an interesting screen name. Oh well to the point. I agree with you that it does seem illogical. As a former U.S. Marine Expert Marksman I can tell you for certain that with expert training you can become an excellent marksman (not an expert marksman) with fewer than 100 rounds if you have proper training on targeting and dry firing before using live ammunition. This is not typical training but time tested expert training that the US Marines are famous for; that type of training would not have existed in the Napoleonic era. Becoming a true expert will take much more. So there is no possibility that I can see where six rounds once a year could contribute to good marksmanship.
On the other hand Didz suggests that perhaps this is not for marksmanship for line soldiers but rather simply familarizing the soldier with loading and recoil. It is clearly established that muskets where not accurate and line soldiers were not expected to aim and hit a specific target simply to fire and reload quickly and hope the ball hit something. If that's the case 6 rounds a year would be sufficient to accomplish that job. Skirmishers who were expected to hit specific targets would need far more than 6 rounds in training to become good marksman.
The other thought I would consider is that the records available for Steph to access for his previous post may have been inaccurate, incomplete or mistranslated from the original documents. That can happen with documents today let alone going back two centuries. So you could be right if that's the case, it could be 6 boxes, 6 pouches etc. We can only know for certain if we have a time machine to go back and personally verify.