Animals, trees, and crops are subject to variable harvest. This means that the number of units they produce each time they ripen may vary. For instance, a chicken may lay 2 eggs, or it may lay 3 eggs whether it is protected or not by the Chicken Coop.
Variable harvest is not the same as stealing. Variable harvest means you still get the full harvest, just slightly differing amounts, while stealing means you lose some of that harvest.
The variability will depend upon which species. Some species, like the Kiwi, do not have a variable harvest. Kiwis will always lay one Kiwi Fruit. Others, like the Star Fruit Tree can vary from 1 star fruit up to 20. Most species will vary only by 1 or 2.
Whether the animal, tree, or crop varies to give more or to give less at each ripening time is random. It is not dependent on the age (or any other factor) of the animal, tree, or crop.