Panzer wrote:You always have a chance to explode when you overcharge a weapon. A dice can't be modified below 1 with positive to-hit modifier.
Not with a +1 modifier.
Another explanation would be that overcharged weapons do not fire instantly, they take a second or two to get into overcharged state, and the longer you wait with firing them the larger is the probability that it explodes.
Warhammer is rly more a game than a simulation. Don't look at it too close...