By default, that's indeed true. But, first, that can be changed via the weapon's 'RandomStun' setting. The original question was about the Stun Rod, and that family of weapons in particular does have 'Randomstun: false'. In fact, every weapon with the 'Stun' damage type defaults to non-random stunning (although electric stunners in XCF are not actually using that type any longer).
What might have been misleading in my original reply is that the general 25% stun is indeed randomised and thus more like 112.5% damage on average. Still more than 105%, though.
Electric damage is a new custom type with no specific defaults, so what it does is up to Solarius. Some, like the stun sticks and tasers, are non-random. Others, like Thunder Arcers, electric darts or built-in alien weapons, are random.
There are even weapons that lie to you about their health damage and do 0-100% of the final damage after the first round of randomisation and armour effects. Mostly bows.