It's a combination of both. Higher reps = more muscle microtears = more to inflame and cause soreness during repair, and capillary density from cardio and conditioning is a veeeery big determinant for soreness as well, because the capillary density gives you greater nutrient flow and oxygen flow so the oxygen debt is not as prolonged, and neither is the build-up of metabolic byproducts. Better circulation = less soreness for a variety of reasons.
The other variable is frequency of training (which plays into conditioning and cardio, but from the weights side). If you only hit a movement or body part 1 time a week the recovery process is very slow and never pushed. If however you start squatting 4-5 days a week and get to push your recovery abilities up, your body responds with better nutrient transport and increased capillary density to keep up with the constant work demands. This is maybe not quite as pronounced as doing cardio, but it is one reason olympic weightlifters almost never get sore (of course the eccentricless action of dropping the bar helps a ton too).