My advice, ignore whatever it says. It does not matter anyways. The biggest reason is that the AI grabbed something or capped a marked zone when you where not looking. You can cream the ai and take all the squares but if one marked zone had ai walk threw it, its basically theirs (or yours) if no one is hanging out around it. Having the marked areas where the ai has grabbed it (or you) does not mean you lost the grid either. It just means that key point is not under your control. Which is true. You are not sitting on it, it is not necessarily yours. And it will only show who is in control accurately if a least 1 of your guys is there to keep tabs on it. it does not magically tell you if ai capped it at some point and no one is there to know.
The other reason can be you lost a bunch of stuff or losses where greater than what the ai had. Or you so out numbered the ai that unless you have like next to no losses it will at best say draw and and worse say total defeat. You will never see win in this case. You simply showed up with some insane force and was pretty much a forgone conclusion.
In the end the win loose message is meaningless. THe criteria the game uses is logical and makes sense. but its basically giving you an efficiency report and not reporting win of loose. After all, you can win and not be efficient with forces or losses. To illiterate, you can loose everything on the map but if you where out numbered and killed way more of them than your combat power shows, guess what? It reports total victory or victory. THe criteria simply do not reflect the reality of win loose. But it does do a good job of reporting effectiveness.
Now with that said you might go why am I being penalized for showing to battle with some epic out numbering force? You really are not. It was not an efficient use of combat power and sort of violated the idea of economy of force. On the other hand, it worked so who cares. But Generals that showed up to the fight with overwhelming force are forgotten in history. The ones that pulled out victory while out numbered and such are. If it where up to me the only win or loose message would be at the end of campaign. That is figured out in a different way that basically added up all the grids you control and sometimes how much you killed or lost.
I really wish that win or lost message was not there and replaced with something else. Like you where 67.2 percent combat effective for given battle as the criteria is pretty solid. Just not the message it says. Anything but silly win loose. Nothing so stupid as to end up controlling the entire battle area and it tells you total defeat or get kicked of the map and say total victory. If that is winning UGH....