Market Value=Share Price*Volume.
Other than that the exact math hasn't been released (at least not to me). It also works on rolling historical values, if your corp is losing money recently but gaining prior to this it will take a few months for this to stabilize. In real life the share price is placed at a position where it will sell, more specifically the price it sold at last. This has no basis in the game, probably because it would be hard to manipulate and there would be lots of cheating involved.
Most likely what I think you are seeing though, is the corp was undervalued, when corps first startup they are always undervalued. When you think about you spend like 10 times the value to start up a corp then what it is worth at its conception. If you like at its balance sheet you will see like 60B in cash and only a 6B market value. That doesn't make sense. So the game will bring it to a respectable level. If your corps continue to lose money the corp will lose value.
So I guess to give a good response the question becomes are the corps new?