The most simple reason that generally applies for many types of organisms is that they thrive in a specific range of temperature. As a result, growing the same organism (usually bacteria) on two Agar plates would result in "different looking colonies," which, in other words, mean that there are more colonies on one plate than the other. This is because when organisms are living in their optimal temperature, the cells and enzymes operate at the optimal rate. The higher their rate of operation, the higher their metabolism, and this generally leads to an increase in number for microorganisms growing on Agar plates.
For example, let's say we want to grow "organism X." It favors living in temperatures ranging from 20-30 degrees Celsius. We plate "organism X" onto two different agar plates, which are then placed at two different temperatures: 25 and 50 degrees Celsius. In this scenario, we should expect more colonies on the agar plate in the 25 degree Celsius environment than the other one because "organism X" is growing at its optimal temperature, which allows for their biochemical components to work at the most efficient pace. Since bacteria likes to grow, reproduce, and spread, when the components work at the most efficient pace, the bacterial cells are reproducing much faster than those who aren't in the perfect environment, which is the 50 degree Celsius. This is why at different temperatures, one would observe different looking colonies.