I suspect that developers originally intended the rate for Greaters to be lower, thereby encouraging people to at least look at taking Impregnation in Alchemy path.

Here's another thing that occurs to me.

If the drop rate is very high, there is no reason to consider taking Side Effect ability.

I built a Swordsman/Alchemy Geralt who went to Impregnation. He got there via Synthesis and Potion specialist. With the drop rates lowered, I would go down the other side, taking Alchemist (bombs and traps) and Side Effect instead.