If you don´t know too much about mathematic, it´s not up to me.
But I will explain it to you, mybe your brain will understand it now:
If 4,9% are 100% of the alcohol in that beer, so 4,8% are not 100% anymore, it´s now only 97,979%. That means, there is 2% less alcohol in that beer, than before.
Got it now?