Damage are the same what ?
Yes damage of most axis/allied grenades is 80. Most Soldiers have 80 hit points with an armor of 1, so standing in the blast radius exactly kills them. Green cover reduces the damage by half if the grenade is thrown from the other side of the green cover.
The difference is the blast radius which is called Area of Effect (AOE). While most short ranged grenades have an AOE of 1.25 the rifle nade only has 0.75. If you nade a 4-man MG team or 4-man squad (like brits) that is clumped up in cover as usual an AOE of 0.75 is enough to hit all four, but in an open field situation a rifle nade will do much less damage than the short ranged grenades with their AOE of 1.25. So upgrading your IS/Sappers to 5-man squads can really make a difference when it comes to surviving grenades, in addition soviet 6-man weapon teams have a higher chance of surviving rifle nades because they are usually a little more spread out.