There are a number of possible causes for the error, but start with using the string method correctly. The string must be run symmetrically to both dropouts. The loop and knot you have on the right side can throw off results. Repeat that measurement threading the string as you did on the left for both sides (through dropout and tied to opposite side. Then measure carefully.

Also confirm the centering at the bridge using a known to be correctly dished with a straight axle. Be sure both ends of the axle are against the top of the dropout. Confirm my flipping the wheel, and reconfirm by rotating the axle with a cone wrench on the left side (a bent axle will push hte wheel to one side).

Now understand what you're measuring. The string method is a quick (but non-difinitive) diagnostic that the dropouts are at equal distance from the central plane. Even if they are, it's still possible for the wheel to be off center at the brake bridge. For example, the stays could be slightly curved, or one dropout may one slightly higher than the other. So go slow before "correcting" anything until you have a good sense of what the problem really is.