IMSTO doesn't make the keycaps himself, he only does the dye subbing on keycaps(usually Vortex in the past, now Gateron). Vortex and Gateron are 2 different brands and use different molds as they are different in a couple ways.
Gateron has lines along the bottom, a stem tightness issue, and 90 degree angle inside edges on the underside of the keycaps.
Vortex has no lines on the bottom, no stem issues, and rounded inside edges on the underside of the keycaps.
Plus the thickness of the keys are different on Vortex and Gateron.