My guess is they are seedlings. They sell them at grocery stores in my area as well, and they’re all imported from Mexico. I’ve had noticeably different types. It’s quite fun to see what type you get. Some have deep orange flesh and are soft and melting, others have bright yellow flesh and are super crunchy. Some of them get horrible rotted patches and never ripen properly. I’ve had some outstanding ones that have really beautiful bright orange flesh, crunchy type, with a tropical bubble gum, mango/banana flavor. All these variations make me think they are random seedlings, or maybe different known cultivars.