So this is what was used:
Treeplanter's Pot 10.10.40 + Soil 10.10.56 + Water 10.10.44 + Herbalist Table 15.12.19 + Red Currant 10.10.10
=> (results verified by "Inspect" action")
7.9.17, 12.10.18, 11.9.20, 8.5.20
12.9.22, 6.7.15, 11.9.21, 5.10.22
As you can see the Substance element in each of the ingredients is at least 10. Yet only 2 of the berry bushes had 10+ substance. This is very weird since if there was a reasonable random factor, there would be a few above 10 and a few below 10; yet the grand majority is below 10. If there was no random factor then the Substance would be roughly 10-11.
This seems to be a bug in how the random factor is added or rounding. (Or I am just very unlucky, but that's unlikely)
Edit: How it looks now it suggests that the random factor can add at least in the interval -5 to +1 to an element.