GOOD subject. I recently answered this on a list or two, so let me dust off the answer I gave there.
Part of the problem is that everyone sees what happens through the filter of their perceptions. So, eye-witness accounts often vary widely...and they are ALL primary sources. If you're reading source A, you will have a very different account than someone reading source B.
Many historical factoids or accounts are actually secondary sources, to begin with. Why? Well, unless you are going back and translating the primary sources personally...AND you have a comprehensive knowledge of the time period, the influences, etc.... Well, even then, it's secondary, because it's a primary source filtered through your secondary eyes, which is why even the secondary sources, working from the same primary sources, do not always agree. A single word changed can make such a difference in interpretation.
Then you add in the politics and religion of the time and their interference with the source matter we have available. The victors write the lion's share of the history, and prosecution, persecution and/or excommunication has gotten many a person's attention over the centuries. When ten accounts say A and one says B, you might have a false lead...or you might have the one person who recorded what could get him/her killed or imprisoned to write.
Worse, when it's within the lifetimes of those involved, some people will doggedly insist their version...the version the spin doctors fed them...is correct, despite what evidence exists to the contrary. (And, no...before someone launches a political debate in here, I am steering FAR away from current events. You can find examples of this in Viet Nam, WWII, and further back...)
So.... No matter what actually happened, there are going to be ten versions of it, at least, floating around, everyone making his/her own spin on the same events. The BEST you can hope for in answer to "What actually happened?" is an amalgam of reliable primary sources that take a middle ground between them.
The best you can do...if what you write must be historically correct, to the best of your ability...is either state that it's alternate history or name ONE source you are sticking to. I did the latter in Black Sail, which is out by Phaze (mythological rather than strictly historical in nature). I named my source as Edith Hamilton and left it at that. Anyone who wants to argue that it's wrong according to historian X has no battle with me, because I wasn't basing it on historian X.