Most contracts can succeed with some distributions and defenses of the 26 opposing cards, and fail with others. If a contract (except a grand slam) makes against any defense and distribution you've underbid. If one never makes against any distribution and perfect defense, you've overbid. The latter two are clearly "non optimal" contracts.
When I learned bridge in the early 1970s, I was taught to "bid to make." With a standard eight card trump fit and 26 points, one should be in a four level contract that would make if e.g. the trump suit broke 3-2 (a 68% chance) but would often fail if the suit broke 4-1 or 5-0. Other equivalent (68%) chances were a combination of 3-3 suit break followed by a finesse.
More recently, scoring rules have changed, especially for IMPs, so that you will come out ahead if your contracts have a 50% chance of making, and in some cases, even 40%. Have bidding systems adjusted to these changes, requiring say, 24 points instead of 26, so that your contract hinges on 50-50 finesses rather than the 68% chance situations described above?