Low bullet damage after it penetrate a glass from inside

In some of my experience , the damage of bullet will be lower when me shooting a guy from inside (building) to outside and the bullet need to penetrate a glass first . What ever the grass it was , a breakable window glass but bug to unbreakable by bullets or a huge glass wall , the damage to shoot outside target usually lower than normal . You will see the health bar of enemy decrease very slow . When the enemy shoot from outside to inside , the target inside will take full damage . So I think the bullet damage of penetrate glass has a  bug .