Stats during evolution

The stats didn't chance when you evolve.

The overall stats change (for example, Venusaur will be stronger than Bulbasaur)

but the individual values (like the 13/15/14 which you are referring to) do not change when you evolve Bulbasaur to Venusaur

Every pokemon species has three base stats (attack, defense, and stamina) that are used as the starting point for stat calculations. These base stats are listed on the pokemon pages here at GamePress.

Each individual pokemon has three individual values (IVs) that range from 0-15. These numbers are added to the base stats during the stat calculations.

When you evolve a pokemon, it changes to another species. Its base stats change, but its IVs will remain the same.