My guess is that it is slightly higher than that because often times the label “asexual” is applied to people who have 0 interest in sex, while others experience a reduced emphasis on sex, hence the “spectrum” part. Also it isn’t something people openly admit to it, so it suffers from the same problem as trying to quantify the numbers of people who identify as bisexual.

And then there's people who the label could be applied to, but spent much of their lives trying to conform to more socially acceptable ideas about sexuality.