The short answer is that this is because the humidifier is working harder.
In dry conditions like cold winters or hot dry climates, more water is absorbed into the air and at a faster rate. This increased water usage is actually a sign that the humidifier is actively humidifying, not a defect, and will deplete the water in the humidifiers tank more rapidly.
In more humid seasons, the same tank may last significantly longer because the air can’t absorb as much additional moisture.