Dear temp67: Asymetry means that the tissue does not look the same throughout the beast. This is not uncommon, particularly in young women whose breast tissue may be more dense. Appears benign means that there are no characteristics to the asymmetry that appear concerning. The recommendation to repeat the mammogram in 6 months is to check to be sure that things appear stable (that the asymmetry looks the same over time and is not growing). If you remain concerned, you might set up an appointment with your doctor or a breast specialist who can explain the results specifically and determine if any additional testing (such as ultrasound) would be indicated.
Often it is the result of some fibrosystic change. Since they don't have anything previous to compare it to they are going to follow up in 6 months to see if it has changed. Sounds like they are doing a great job of taking care of you. I have breasts that are prone to fibro changes. They tend to have lumps and bumps that are benign, but still are lumps and bumps. Be glad they are being cautious.