I don't believe there is anything missing here Jonni. When purchasing stock, the corp purchases based on 100% production. So in a scenario where:
100% production = 100 services/month
120 months = 12,000 services
corp produces at 127% = 127 services/month
12,000 services - 127 services (1 month production at 127%) = 11,873 services
corp calculates 11,873 services= 118.7 months left
However, I would guess if more months are missing, the stock was purchased with less than 100% employment. I believe that the corp adjusts supply purchases based on the current employment level.
So, in the original post. If the corp has say 90% employment (due to a labor situation), the following would happen:
100% production at 90% employment = 90 services/month
120 months at 90% employment = 10800 services purchased
1 month production at 90% employment at 127% production level = 114.3 services used
10800 services - 114.3 = 10685.7 services left
If then, labor situation is resolved and 100% employment is achieved.
10685.7 = 106.85 months of stock
There you go. The longer explanation. This is why examples are needed. Which corp? etc. I would guess that the corp that went from 120 months stock to 112 in one month went from 95ish% employment when the stock was purchased to 100% employment the month after. If only I had the corp to look at..
On a completely different note. Stocking corps with large amounts of supplies is risky for this reason. If the corp is lost due to natural disaster/war/bankruptcy/whatever there is no compensation for the lost stock. It is gone. In short, I would not recommend such large stock levels. A proper buying strategy for supplies can be sufficient to avoid shortages and carries much less risk.