well in this case i think the female became pregnant before you got these and it slowly developed for roughly a month before being born. that or these snails are able to store sperm for some time. it is unlikely that a tylo under one inch is able to reproduce. so this happened in the tank of the seller (was it alphaprobreeders by any chance?)
If you were going to say that your other snails were a different species tylo (and adults, not juveniles) then I would argue that your female snail crossed with another species to make that baby. hybridization seems to be possible with tylos.