This is cause corporation value in this game is tied to earnings per share and P/E ratio, and its a VERY LONG (too long) delay (2-4 game years). I've already complained about this once.
I've actually had corporations that were profitable for 3-4 years straight still sinking in value (to below 10B in a few cases), because their P/E hadn't fallen below about 50ish yet. On the flip side, I've had corporations losing money for a couple years and still gaining value, due to their P/E not going above 50ish. I mean I understand the desire not to have P/E react to sudden profit changes, but the current implementation is extremely poor, the delay is making value not match reality whatsoever. In a few cases I've had corporations have a large loss, then a large profit, then a large loss, then a large profit again (over say 4-5 years), and NOT ONCE rise in value (military supplies on WG is an example, I can surely find others).