every zombie is assigned a ‘half-life’ value in days (i think standard zombies is 28). this value is the point at which half of the population of that type of zombie is expected to be evolved into the next type. some will do it earlier, some later, but that number is the mid-point or ‘half-life’ for that population to be upgraded. The evolution scaling factor is a multiplier for that half-life value. so at default 4.0 evolution the standard zombie has a half-life of 112 days (4x28)…again some WILL evolve sooner, sometimes much sooner, so you can find upgrades early, even day 1. if you want SLOWER/LESS evolution then increase the evolution scaling factor. if you want FASTER/MORE evolution then turn the scaling factor down.
TLDR: no matter what you set the evolution at you CAN still get upgraded zombies right at the start. Random be Random.