Likely both upper eyelids have central levator partial disinsertion. This is the tendon that raises the upper eyelid. Normally this tendon fans out and inserts across the upper eyelid tarsus. This tenon can and often does slip from its insertion centrally. Before it gives rise to upper eyelid ptosis, it causes a change in the position of the upper eyelid crease and induces a compensatory eyebrow elevation. Some call this latent ptosis. At some point and often after blepharoplasty, this tendon slippage can actually be the basis for the development of upper eyelid ptosis. Here it is the basis for your upper eyelid asymmetry. The fold is composed of skin and two fat volumes. One fat volume is contributed by the fat that lives under the skin and orbicularis oculi muscle under the eyebrow. The second fat volume that contributes to the upper eyelid fold is the pre-aponeurotic fat that lives behind the orbital septum. When the eye is open the septum relaxes and this volume contributes to the upper eyelid fold. With slippage of the central aponeurosis, the pre-aponeurotic fat also slips with the levator. That means less fat is available to contribute to the fold. Also when there is a compensatory eyebrow elevation, less sub-brow fat is available for the fold. That is why one eyelid does not have an apparent fold. Letting an eyelid plastic surgeon remove the normal fold (and this is what is commonly offered in these circumstances), both eyelids will be skeletonized without the correction of the underlying central levator disinsertion. The surgery does not result in a correction of the compensatory eyebrow elevation so asymmetry will remain. An alternative repair would be to perform crease lowering on both eyelids with a repair of the central levator tendons and an anchor blepharoplasty. I would counsel you to have the eyelid missing the fold corrected. The presence of an outie fold makes for younger looking eyes. If for some reason you did not like the effect, you still have the option of removing the fold skin but making a fold is much more challenging.