It’s actually pretty common for the bumper or fender measurements to differ slightly even when both coilovers are set to the exact same length. Body panels are rarely perfectly symmetrical, and small variances in the bumper, fender fitment, or even worn bushings can easily create a 1/4"...
Did you corner-balance the car after raising it, or did you just match the coilover heights and call it good? Also, have you checked if the preload or spring seating is identical side to side? And is the car on completely level ground when you’re measuring the fender heights?