Anything beyond or different to a just action is unjust.
Shakespeare's "King John I"
If you try to make something "better than well", you won't be able to make it at all. (overestimating your skills)
There is a way to make something well. Trying to make it "better"/differently (often) leads to disaster. (overzealous is regarded problematic)
The saying does not refer to not try to get better, but that we should have as a goal the good, and that good is enough. If we try to go beyond good, often we will not find the better but will simply move away from the good and we will do the mediocre or even the bad.
In pursuance of the analogy between the virtues and the arts the moral
idea πλεονεξία(overreaching, getting more than your share; see on 359
C) is generalized to include doing more than or differently from.
English can hardly reproduce this. Jowett's Shakespearian quotation
(King JohnIV. ii. 28), “When workmen strive to do better than well,/
They do confound their skill in covetousness,” though apt, only
illustrates the thought in part.
When workmen strive to do better than well,
They do confound their skill in covetousness;
And oftentimes excusing of a fault Doth make
the fault the worse by the excuse,
As patches set upon a little breach
Discredit more in hiding of the fault
Than did the fault before it was so patch'd.