Why is it that my charger that has a rating of 100-240 volts and 1.3 amps can handle/input more watts at a higher voltage when doing the formula watts=volts x amps? In addition, why can't it, at a lower voltage, handle the same amount of watts that it can at a higher voltage?
Simply put, it does not.
It will deliver its rated output while being able to use a supply within the range specified.
If the voltage is at the high end (230V) then the required input current will be lower, if the input voltage is at the low end then the current goes up.
Your body can only process a limited volume of food (current). If that food is more calorie dense (voltage), then you can do more work. If that food is less calorie dense and you've already eaten all you can eat, then you can 't do only do less work since you are unable to eat more food to compensate for the lower calorie density since you can't just eat more food to compensate. Your charger is the same.
Excessive current causes heat dissipation as well as causes saturation in transformers and inductors which all limit power output. Your charger can only handle a limited amount of current. If it's already running at maximum input current and you lower the input voltage, it has less input power because it can't increase the input current to compensate.