Technically speaking, maximum efficiency can always be at 100 % . comparing this fact to the growth rate of hay grass in rimworld, I observed growth rate to be 140%. this is only for that specific haygrass. neighbouring hay grass's growth rate is 100%.
Why is that hay grass growing at 140%?
Edit: earlier i had thought that was a bug. But from the answer below it is clear.
I am using vanilla rimworld 0.14.1234