I have never found this to be the case. Is your view based on experience or are you supposing?
Are you referring to my claim about the element of surprise? I suggested it because I see it as an environmental factor that is generally out of the control of the martial arts practitioner that also has a significant weighting on the outcome of a violent confrontation, and one that is not trivially compensated for with training. I don't base it on personal experience (I'm lucky enough not to have had to put my myself to the test like that), but it seems intuitive to me that should I be surprised by a blow to the head, I would not be confident that a few years' training would be enough to outweigh that disadvantage (though in a sport like boxing perhaps it would be).
The initial disorientation from an attacker's first strike would make it difficult to assess what was happening quickly enough in order to engage in an appropriate defensive behaviour before being struck again and then repeatedly. People that have not been trained to receive landed punches are generally not well-equipped to handle them, and attacks can happen lightningly quickly - before you know it, a person can be upon you, levying a series of blows that have left you injured.
I wouldn't identify the element of surprise alone in trumping a person's initial forays into martial arts, but also weapons, body-size, etc. In my opinion, it seems likely that it would take a fairly significant amount of training to surpass these relatively simple environmental advantages an attacker may have.
As I say, however, I'm not speaking from my own experience of being attacked, only from how confident I personally feel in my own self-defence. If I'm off the mark then at least I suppose it's better to underestimate oneself than overestimate, and I appreciate your views on the subject.