Two identical cars, same mileage, same history. One presents well and one does not. The gap at sale is consistently larger than the cost of closing it.
Buyers read condition as maintenance
Nobody can assess an engine in a ten-minute viewing, so buyers use what they can see as a proxy. Swirled paint, a stained interior and a dirty engine bay suggest neglect everywhere else, whether or not that is true. Presentation is doing evidential work.
Originality is what holds value
This is why paintless dent repair and paint protection film return so well: both preserve the factory finish. A repainted panel shows on an inspection report and takes money off, even when the repair is good. A dent removed without paint does not.
Timing
Correction and interior work done shortly before sale return most directly. Protective treatments return over years by preventing the damage that would otherwise need correcting. Both work; they just pay out on different schedules.