Take a dice with 6 sides, it's a 50% chance of rolling an even number on any give try. Every roll will give you a 50% chance but, what's the chance of rolling 3 odds in a row or 4,5, or higher? So what would be the chance of rolling 100 dice and getting all odds? Much lower than 50%.
Another thing is the anvil does go in cold and hot streaks.
Good Luck testing out your upgrading.