They were probably already pregnant when you bought them. Either that or one is a male and you don't realize it. If you can provide a decent pic of them, we could try to see if one is a male.
All female livebearers can hold onto sperm long-term after one mating, which means they can impregnate themselves, with or without a male, several times afterward. This is a survival mechanism for times when males may not be around to continue the line. Most likely this is what is going on in your tank.