Outdoors, humidity rarely stays the same. It will be high in the morning, and lower in the afternoon, in most cases. Your animal's cage can do the same thing--let it fluctuate from 50% to 80%, and consider 60% to be an average to shoot for, not something you have to keep dead on stable at all times. Variable humidity is likely healthier for the animal, and allowing the cage to dry somewhat, periodically, will help reduce mold or bacterial issues.