Percent yield can be less than 100% if the amount produced is less than the theoretical yield expected based upon the math.
Let's imagine we are making double cheeseburgers.
1 bun, 2 patties, 2 slices cheese
1 tomato, 2 pickles, 1 squirt special sauce.
We are given 20 buns, 40 patties and 30 slices of cheese along with an excess amount of all of the other ingredients.
The 20 buns will let us make 20 double cheese burgers.
The 40 patties will also let us make 20 double cheese burgers.
However, the 30 slices of cheese will only let us make 15 double cheese burgers.
The cheese limits our production to 15 double cheese burgers.
This is our limiting reactant and theoretically we can produce only 15 double cheese burgers. Our theoretical yield = 15 double cheese burgers.
During preparation 4 slices of cheese fall to the ground and need to be thrown away. This leaves only 26 slices of cheese and now we can only make 13 double cheese burgers. This is our actual yield.
We can actually produce 13 double cheese burgers.
To determine our percent yield we divide the actual yield by the theoretical yield.
% yield = actual yield / theoretical yield
% yield = 13 / 15 = 0.8667. or 86.67%
Our % yield is less than 100%.