I have been reading Morgan Llywelyn's Grania: She-King of the Irish Seas
This aspect, the repetitive explanation, leads to the third thing historical novelists do, and that is where speculation comes in. I truly believe that this is both why people read historical novels instead of biographies and is also the hardest part of the craft. It is our job to look at the person and the facts and illustrate what we guess is the motivation for what happens. Why did Grania keep risking disaster? In other works, why did Llewellyn the Great forgive his wife Joanna’s infidelity? Why did Raymond of Toulouse desert during the Battle of Merzifon? Why did Elizabeth I stay unmarried? So often history offers us hints, but rarely does it give us definitive answers. Even when a historical figure writes down why s/he did what s/he did, can we trust that person to be honest with him/herself? In this novel the author has a big task. The historical "Grace O'Malley" is a tangle of contradictions. She appears in the historical record to switch loyalties and to betray her own people, coming back to them and fighting for them, only to switch again. One might speculate whether the woman was no more than an opportunist, or, like Llywelyn does in the novel, find other motivations, individual motivations, to ascribe this peripatetic life. That's where the book shines. You do see the speculative character based on a real person grow and you understand better her particular choices.
Those who rail at a historical novelist for an interpretation of events usually miss the point. The point is we don not know for certain. I have long believed that fiction writing is a sort of more humane form of the science of psychology. We write, and others read, a novel to examine the facts, add what we know of the human heart and mind, and we explain, suggest, and illustrate motivations and feelings. It is creative writing, but more than that it is speculative writing. It is storytelling that suggests "Perhaps?"