This morning I built a 5,200 SF virtual house in Houston. My cooling-load calculation indicates this house needs only a 3.5-ton AC system. Based on various comments I have read on this site, there is no way I could cool this house with less than 10 tons. What have I done wrong?

The house has two stories of equal square footage. Inside dimensions are 35 x 70. The first floor has 10-foot ceilings, and the second, 9-foot ceilings. Five bedrooms. Roof is tile. SF of windows and sliding glass doors: N - 213, E - 75, S - 250, W - 75. All glass is low e and has a SHGC of 0.33. The house is well built and very tight with a measured ACH of 0.3. It is assumed the owners will close the blinds on the east and west windows during hot weather. All glass on the south side is shaded by eaves.