To modify means to change. You might modify a travel itinerary such that you're leaving a day ahead of schedule. Unmodified means "changed in no way" — you have made no modifications.
Check out any recipe on line, and you will see in the comments cooks raving about how great it is, followed by their laundry list of modifications: "This roast chicken was amazing, but instead of chicken I used tofu, and instead of roasting it I ate it raw." No one feels like commenting on an unqualified and unmodified success.
