Analysis / Mismeasurement
"Under" is almost always a safer option than "over" when in a situation in Real Life that could possibly involve this trope. For example, if you're not sure you took that sleeping pill, and you assume you did and do not take another, the worst you will have is another night of insomnia - whereas if you take what turns out to be a second or third "to be sure," you could end up dead. If you assume one squirt of lighter fluid isn't enough for the BBQ, you can always add another squirt if it doesn't light - whereas if you squirted half a bottle onto the BBQ, you may have a fireball and explosion rather than a lit BBQ. This is also why it's not a good idea to keep medication or alcohol -or worse, both - at your bedside, because it's too easy in a sleepy/tired/confused/drunk state to accidentally invoke this trope and overdose.