There's a bit of a skewed concept of personality.
To begin, people equate others to themselves to determine motives, thoughts, and the like. They see a snapshot of a person- his actions and what he says aloud, but never his innermost thoughts- and assume based on these, using themselves as a control group, his personality. People fall prey to this and the fundamental attribution error and in turn, form the wrong idea of somebody's personality, thus resulting in errors affecting any social experiments on the consistency of personality.
Secondly, what exactly is personality? Imagine people as this: inside is a core of incredibly basic traits, solidified at birth, constant and near impossible to change. Engulfing this core is a fluid sea of more complex, specific traits. These are most susceptible to the environment and are likely to change. So then, do we define personality as the core or the sea or both? If we defined by the core, then one can assume personality is largely fixed. Even if we defined personality by both the core and sea, while one can and will change the outer specific traits, the most basic traits of one's core stays consistent, so in that case is personality fixed or changeable?
Humans are complex, thus so are their personalities. It is multi-tiered and undefinable by one word. Like nature vs nurture, both fixed and fluctuating qualities are present in personalities; it's simply a matter of determining and sorting which traits are of the fixed and which are fluctuating. This is the true question when endeavoring to evaluate one's motives.