Why assume the lower aircraft was exactly at 19000 feet? Maybe he actually was at 19080 feet.
Even the tighter RVSM standard allows an "altimeter system error" of ± 80 feet, and an automatic altitude control system error of ± 65 feet. Toss in the difference between the static port "altitude" and the top of the tail or fuselage (reflective surface), and the lower plane might easily (and legally) been flying with its highest radio reflector at 19080 feet, 1920 feet below you.
Or perhaps both aircraft were within the margin of error in opposite senses: You were 40 feet low (but legal) and the other plane was 40 feet high (but legal).