eustation tube dysfunction--the holes bigger?

my 2 year old had eustation tubes placed in july 08. 6 months later, she had lost the tube in the left ear and had to go under anesthesia to have it removed. the ENT found that the hole was quite larger than the one she put in to place the tube. now, my daughter has a hole that's 30% of her eardrum, that may or may not heal on it's own. does anyone know how or why this happens? the ENT was vague about it and only said that she had only seen this happen in 3 other cases in her career.