As far as I know, there are two decimal places or more kept track of, but only one being shown. The rounding happens at the second decimal place or deeper. The display is shown rounded to 1 decimal place, but that is different than the actual under-the-hood arithmetic to reduce the perception of rounding errors.
Also, I understand the base growth rate to .3 not .25. And the corresponding 10% would be .03. A 5% increment seems to be the smallest they typically use and it would be .015, which leads me to suspect they track down to three places and just don't add clutter to the display about it.
There might be a confusion about the %growth bonus term used in the game. The percentage is not applied to the population, but is applied to the base growth rate instead. Therefore. 100% bonus is .3 base rate plus .3 bonus, giving .6 population points per turn. 10% bonus is .33 population points per turn total.