Because nobody knows!
She allegedly has XY chromosomes. Which are the chromosomes you would see in a biological male (females are XX) But very very occasionally a person is born with XY chromosome but female external genitalia (labia only, no cock and balls). So they are brought up as female, despite having male chromosomes. People think this might be the case because she was allegedly declared female at birth but allegedly was found to have XY chromosomes when tested by the boxing commission.
She has also previously allegedly been found to have high levels of testosterone, for a woman. A sex hormone that is in men and women but much higher in men. It causes male physical characteristics : hair, muscle mass etc That could be due to her having XY chromosomes, or some other reason. A biological woman with freakishly high natural levels, or a biological woman who is doping (taking testosterone injections)
Does that clear it all up?!