He believes the difference is in the in the touching, The lawn cutter might just as well not have been there at all; the gardener will be there a lifetime.
With this statement, granger aimed to convey the importance of legacy. The lawn cutter represent the result that does not leave a legacy, because eventually the result would be undone overtime.
The gardener represent result that leave a legacy, even if the result is undone, other people would redo it again to keep the legacy.