That's interesting, David, thanks!  It fits with the general psychological tendency to overestimate how much people agree with you and suggests that the feeling of besiegement often expressed in introductions to philosophy articles is to some extent rhetorical flourish (though there may be big individual differences in feeling of besiegement or consensus).

I'm trying to think what anti-realism, internalism/externalism, and idealism might have in common that explains their being outside the norm in that way, but nothing leaps to mind.

On the issue of individual differences, one could look for correlations across questions: Are those who overrate (or underrate) agreement with their own position on one question more likely to overrate (or underrate) agreement on other questions too?