You should get a mistaking :P

Just kidding. IS your current temp/humidity gauge an analog gauge or Digital? I only ask because the analog gauges are as far from inaccurate as you can get without being called something else.

The HydroTherm will be way more accurate for your enclosure. If the soil is moist and not dripping wet and you have 50% to 75% of the top covered I would venture a guess and say I bet your humidity is in the 70% range and not 30 - 50%

What are you covering the top of the tank with? Have you tried using a damp towel to cover the tank. It won't absorb the humidity and helps a ton.