Don't know about Eastern poisonous snakes, but I heard that this is true of rattlers. Normally, they use their venom for killing prey. It wouldn't be good to waste it, as it takes time to produce and they might need it again sooner than expected. So in an "offensive situation" they judge the weight of the prey and inject enough to kill it, but no more.
But in a "defensive situation" when they are confronted by a dangerous predator - including humans, their bite is a quick "fight or flight" reflex - perhaps even a panicky one, rather than a carefully planned attack. As such, they don't have either the time or the wits or both, to calculate how much venom to inject. So on the one hand, you might get a bite with no venom at all. Or on the other hand, you might get the entire sac of venom injected into you. Flip a coin.