It's not speed damage. If a worm stays slightly below a dying one (about 10 pixels), you can inflict more than 25 grave damage. 28 is the maximum.

Nice to know, the maximum I have ever seen was 26 too.

doesnt that works like this?  

i thought that was speed damage D:

check the rep, bar

It should be noted that the center of the explosion often doesn't match the position of the weapon. A good example is the mine, you need to place it on the worm's head to inflict maximum damage. If you place it "inside" or next to the worm the damage is a little smaller.

It's the same when it comes to grave explosion. The dying worm has to be a few pixels above the victim in order to inflict more than 25 damage. But we rarely see it happening, because it's not possible for one worm to die standing on another (it always "falls" inside the other worm's collision mask). So the only time it happens is when the worms are standing on a steep slope with some land supporting their positions.

The effect is also very prominent when you use HHG. Take a look at the replay attached. You should note that the worm that took most damage was Grit (96), because he was the closest to the center of an explosion. Colin was the closest to the position of the HHG, but took only 79 damage.

Why is the centre of the explosion of the HHG not coming from the centre of the position of the HHG but below it?

I don't think there are any sophisticated worms physics in the works here, the weapon was simply programmed to work like that. Just remember that if you want to deal maximum damage with the HHG, you need to place it about 35 pixels above the target.

I think everything is described on the NNN forum, but I don't see any info about grave damage.

P.S. Looks like some pictures don't work anymore :<