Double check all the measurements. Make sure that the string is only around the tube and not affected by any welds or lug flanges. If it's symmetrical at the top but not the bottom the wheel is out of plumb, ie. not vertical. There are many possibilities so you have to check everything.

If you own a bubble level and a repair stand you can do some more measurements. Mount the rear wheel, clamp the bike in the stand and orient the frame so both the seat tube and top tube are level per the bubble level.

This establishes the central plain as level to the earth. Now place a broomstick or any straightedge across the wheel on a secant so it touches the rim but not the spokes or tire. Since the wheel should be in the same palne as the bike, this should be level in all orientations. If it isn't you can figure out how to file the dropouts to correct.

BTW- If you lack a repair stand the same can be done on the floor using books, or other stuff to support the frame so it's level. To reduce the height needed remove at least on pedal, and turn or remove the handlebar.

This test can also be done in a vertical plane with a right angle bubble level, but front/back and top/bottom have to be analyzed separately, and it's a bit harder. If you have a good head for spacial relationships, all kinds of measurements are possible with the bubble level, string and straightedge.