An 1D10 weapon does 1-10 points of damage. A 2D4 does 2-8. So though the max is higher, the minimum damage is also a bit lower.
There is however also a bit on statistical distributions involved here. If you just roll one die any outcome is equally likely, but when you roll more dice than one the outcome is more likely to end up nearer the middle of the range. thus on the 2D4 weapon (mace?) 2 and 8 are less likely to occur than 4 and 5.
|