That suspiciously high 20% doesn't just include people who are LGB or identify as T. It originates from a poll commissioned by GLAAD, who also included in that category anyone who described themselves as "gender non-conforming". Which could mean anything really: clothing styles, haircuts, hobbies, etc.

Some sexologists do consider this to be related to sexuality. For example, Anne Lawrence, who has considerable expertise with gender dysphoria, has noted that in many cases it appears to be a type of 'erotic target identity inversion', in which the subject of sexual desire is oneself in a different physical form - in this case, the opposite sex.

It seems that as more people start to absorb the ideology of gender identity that is being pushed upon society, some who wouldn't have otherwise considered it are applying these concepts to themselves.

Hence the trend of people identifying as 'non-binary' in recent years - it's the logical conclusion of believing that 'man' and 'woman' refer to a collection of gendered stereotypes, rather than a description of one's sex.