It turns out that our daughter appears to have a healthy heart and likely does not have hypertrophic cardiomyopathy. Her original echocardiogram showed a heart wall thickness of 9mm. Although she didn't have any symptoms of reduced blood flow, the measured thickness at 7 mos., was an indicator of potential problems later in life. The last echocardiogram we had showed a heart wall thickness of 7mm at 9 mos.
I feel that the cause for her diagnosis is probably due to the use of the echocardiogram at an early stage of development that hasn't yet been as extensively researched as other stages. My daughter's heart morphology perhaps just hasn't shown up enough to become a statistical possibility on the medical charts that relate the distributions of heart wall thickness as a function of age.
My daughter appears to be proof that heart wall thickness can reduce as the heart develops! She's still taking a beta blocker, because the Dr isn't sure whether the reduced thickness of her heart wall is due to her use of that medication.