An anecdotal observation isn't going to quantify exactly how much louder one gun is vs the other.
Shooting under a concrete canopy vs open air would make all the difference. Unless we are given every significant detail in exacting manner, there's no way other than guessing what you might be experiencing.
Same ear plugs, same range, same day, with the exact same lot ammunition, with decibel meters set up, and record the difference. Then we just might be able to say more, which all that data would anyway.
One difference is that the CSA vz58 may be using a different flash hider that throws more noise forward. The typical issue AR15 flash hider throws more gas up, to keep the barrel reaction minimized, and that causes sound waves to have another 90 degrees less travel to the ear.
When I built my AR15, I deliberately chose the Black River Tactical because it was shown in testing to be 3-5 decibels quieter than an issue flash hider at the shooter's ear.
Put it under a concrete canopy, and all bets are off. Like the difference between a saxophone and clarinet, how the flash hider is designed alters the sound.