In all of my research studies, beginning with my first research study "Enneastyle: The 9 Languages of Enneagram Type" in 1995, the Type 6 consistently struggled with confirming their true type. They identified with all 9 Enneagram Types. The more counter-phobic 6 would initially type themselves as Type 8, the shy introverted and smart 6 routinely typed themselves as Type 5, the perfectionist Type 6 would type themselves as Type 1 and so on.
When interviewed, the 6s regularly state that, "It depends on the situation" when making a decision rather than just answering a question with yes or no. Not only do 6s have trouble identifying that they are the type 6, they tend to see themselves and others in contradicting terms. This is why the Type 6 is often described as the Devil's Advocate. They also tend to track behaviors rather than motivations in an effort to be safe from potential hidden agendas or being mislead...
The Enneagram 6s also state that they tend to choose the type they think they should be, could be, want to be or have become. In general, they report that they will answer the questions in a "inconsistently" consistent manner. Their reason is that they know they can answer yes sometimes and no at other times, yes....again, "It depends on the situation". ;)
In my most recent study "Enneagram Type, Tritype, Instinctual Type and MBTI" the 6s often used the vocabulary of the Type 6 such as smart, cautious, loyal, dependable, and avoided dangerous people or situations yet still self-identified as other types. When interviewed they were able to discover that they were in fact, a 6 once they could hear themselves repeatedly stating that the phrase "it depends in the situation".
In addition, 6s also state that they are wary of tests in general and tend to be somewhat evasive because they are not sure how their answers will be used. As a result, they may try to "fool" the test. Also, they tend to answer questions with the answer "sometimes" because don't want to be too extreme. The often feel that a yes or not answer without a qualifier is too declarative. This suggests that the Type 6 is trying to balance extremes.