Sample (4d4):
Only if all 4 dice show a 1 you can get a 4.
Only if all 4 dice show a 4 you can get a 16.
Now look at all the possible combinations to get a 8 or 10...
You can fake a d13 without bell curve by rolling a d20, subtract 7 and have a min roll of 8 on the ENWorld roller. No multiple dice, no bell curve. But another die for high or low works.
BTW, d30: d10 and d6 [d6 for a low (1-10) 1-2, med (11-20) 3-4, and high (21-30) on 5-6).
(1-2 add 0 to the d10, 3-4 add 10, 5-6 add 20. Every possible number can only rolled by one combination)