How can this happen?

I am growing from an unknown strain that I pulled out of a bag. One of my plants turned out to be sativa and the other indica. How could this or can this happen? Maybe I mixed some of the seeds up or something but I highly doubt it. Any opinions? Has anybody ever had this happen before? I thought it was pretty wierd because I thought that all seeds from one plant would be the same but maybe im wrong. Can you get some sativas and some indicas from the same plant, is what im basically asking?

happens all the time as swag is a mixture of whatever strains grown and harvested together

Oh, now I see. Had no idea. Thanks for the info. Atleast now I know I have one of each. lol