I did a quick search for "dice bell curve"; the first few paragraphs here seem like a decent explanation:
http://oldguygaming.com/probability-and-dice-rolling
it applies to coin flips as well as dice rolls (a coin being a 2 sided dice). with 30 coin flips there are many combinations with 15 heads + 15 tails but only one with 30 heads.
so given 2 ways of calculating the result:
- flipping 30 coins and adding all the heads
- rolling a single 31 sided dice (0-30)
it could be said that the second method is much more random (as extreme results are far more likely)