The Germans did the same thing with the number of Russian tanks and infantry. I don't remember the actual numbers at all but even after having, by their (the German's) calculations, destroyed every Russian tank two and three times over, they *still* would assume that the Russians were now running out.
I would imagine it is just the nature of things when you have to report to a superior you fear or because you can't accept being wrong. Heck, here in the US, each and everyday decisions are made based on the numbers some nose-bleed-level-manager *wants* to hear rather than the actual costs.