There was originally only one shirt that was not identified as male or female. One of your fellow commenters asked if there was a female version of the shirt available, hence the inclusion of a shirt with that description. The "men's" shirt is a standard American Apparel cut. The "women's" shirt was described as such by teespring and seems to have been designed for a smaller framed human. Women should feel free to order the "men's" shirt and vice versa.
or if the men's sizes were bigger than the woman's sizes. how extremely sexist to not offer the same sized shirt for both sexes. disregard biology we want equality at all costs.