Trump's version of the truth is whatever feels right to him in his head at the time. This is classic among those with Narcissistic personality disorder. I'm usually not a fan when people try to armchair psychoanalyze public figures. I have some familiarity with this though, and he's such a classic case that it's remarkable.
Here are the DSM-V criteria for Narcissistic Personality Disorder:
-Having an exaggerated sense of self-importance
-Expecting to be recognized as superior even without achievements that warrant it
-Exaggerating your achievements and talents
-Being preoccupied with fantasies about success, power, brilliance, beauty or the perfect mate
-Believing that you are superior and can only be understood by or associate with equally special people
-Requiring constant admiration
-Having a sense of entitlement
-Expecting special favors and unquestioning compliance with your expectations
-Taking advantage of others to get what you want
-Having an inability or unwillingness to recognize the needs and feelings of others
-Being envious of others and believing others envy you
-Behaving in an arrogant or haughty manner
In short, actual honesty requires some humility and the ability to see things from another perspective than your own. What's true for Trump isn't what's factually correct in reality, but what's true for him. Once you recognize this, his behavior is much more explainable and even more predictable.