What is true about commonly available swordtails is that some males will try to hide their true gender from the other fish by not growing the sword shaped tail.
the sneaker males can stay hidden for quite a long time. A few years back I bought 6 juvenile swordtails that appeared to be females. Each was well over 2 inches in length when I bought them. About 6 months after the purchase, I noticed one of my females was a male. I kept an eye on my fish and hoped that the newly discovered male would do his job of getting the remaining females ready to drop fry. As time went by, I started to doubt his ability with the females. At over a year after my purchase, 2 more of my females revealed themselves to be males. Now I was faced with an equal number of males and females, something nobody thinks is a good idea, but I decided to leave all the fish together and see how things turned out. I never saw any excessive breeding attempts as we need to watch for when there are too many males to the number of females. A few months later, at about 2 years old, I saw 2 more females show signs of being males. Now I knew why there had not been excessive breeding pressures. When another 6 months had gone by, my last female revealed himself to be a male. Now I was certain why no fry had ever been seen in the tank. My males were all fine looking healthy specimens but none of them were as confused as I had been about the gender of the other fish they saw present. They knew, long before I did, that the other fish in their tank were all males. I finally sold my males at a fish convention as mature and really stupendous looking males, not as females.
