crop purity, confused

I am slightly confused by these results:
I planted 100% cereal seeds on 92% humus. Resulting crops ranged between 71% and 76%
I planted 97% cereal seeds on 93% humus. Resulting crops ranged between 81% and 86%
A fellow pilgrim planted 100% seeds on 100% humus. Resulting crops came out only 65% pure
I fail to grasp any consistency in these numbers. Just how big is the random factor?
I planted 100% cereal seeds on 92% humus. Resulting crops ranged between 71% and 76%
I planted 97% cereal seeds on 93% humus. Resulting crops ranged between 81% and 86%
A fellow pilgrim planted 100% seeds on 100% humus. Resulting crops came out only 65% pure
I fail to grasp any consistency in these numbers. Just how big is the random factor?