Population and growth are rounded to the nearest decimal place. Therefore, a 0.25 growth rate instead increases population by 0.3. Each 10% Growth bonus increases Growth by 0.025, which is generally rounded down.
0-30% Growth = +0.3 Population
40-70% Growth = +0.4 Population
80-110% Growth = +0.5 Population
etc.
Assuming this is not working as intended, an easy fix would be to go back to using 3 decimal places for Population.