some plants hermie due to unstable genetics. ive had it happen more than once. if its bothering you try some 'Dutchmaster Reverse'. just spray it on and it'll stop it. i occasionally use it as a precaution, its handy to have it available.
Where did you get the seeds? Strains get watered down by amatuer growers and passed off as "pure" because no one wants to take a hit on a mistake.
Happens all the time. I would say don't blame the pheno, blame the seed supplier