and in ToB it was established that his final letter to you was a lie, so he's not a reliable source of information.
Bob, can you detail this a little ? I don't remember where or when it was established such a thing...
At the end of your second visit to Candlekeep in
BG1, if you manage to keep away from the guards long enough to get to Gorion's room, you find his final letter to you. Here is the text:
[Gorion's Note]Hello <CHARNAME>,
If you are reading this, it means I have met an untimely death. I would tell you not to grieve for me, but I feel much better thinking that you would. There are things I must tell you in this letter that I might have told you before. However, if my death came too soon then I would have never been given the chance. First off, I am not your biological father, for that distinction lies with an entity known as Bhaal. The Bhaal that I speak of is the one you know of as a divinity. In the crisis known as the Time of Troubles, when the Gods walked Faerun, Bhaal was also forced into a mortal shell. He was somehow forewarned of the death that awaited him during this time. For reasons unknown to me, he sought out women of every race and forced himself upon them. Your mother was one of those women, and as you know, she died in childbirth. I had been her friend and on occasion, lover. I felt obligated to raise you as my own. I have always thought of you as my child and I hope you still think of me as your father. You are a special child. The blood of the Gods runs through your veins. If you make use of our extensive library you will find that our founder, Alaundo, has many prophecies concerning the coming of the spawn of Bhaal. There are many who will want to use you for their own purposes. One, a man who calls himself Sarevok, is the worst danger. He has studied here at Candlekeep and thus knows a great deal about your history and who you are.
And yet in
ToB we learn that your mother was in fact a priestess of Bhaal who intended to sacrifice you. That ret-con means that not only did Gorion hide the truth from you all of his life, but lied (or at best offered a half truth) even when he knew he was facing his death.
The real reason for this is that plot changes were made between
BG and
ToB, but in-game it looks like Gorion was a completely unreliable source (unless the Solar and the Wraith are lying).