I would venture to guess that this tree was grown in a somewhat shaded part of the nursery. Once a leaf has matured it cannot change the amount of light it takes in as it is determined during leaf formation. Newer leaves set the size of the openings that allow sunlight in as it's surrounding dictates so will do better in more sun than the old leaves. Also, don't put any fertilizer on any small tree until it is well established, usually the next season . How is the water draining under the tree? Citrus should be watered deeply but less frequently than your lawn to make good deep roots that don't suffer from the heat as much.

I agree with solar. It definitely looks like the leaves are sunburned. When you fertilize next year, I would suggest an organic fertilizer as this type will not pose a risk of 'burning' the roots (among other benefits). Synthetic fertilizer burn(among other problems) is always a risk. Especially in a climate like ours. Not to mention the cost, which will continue to rise with the cost of fuel.

It's hard to tell for sure from the pictures, but I don't see any mulch around the tree. This would greatly benefit all your trees. Mulch conserves moisture, provides nutrients over time, and helps keep the soil underneath up to 10 degrees cooler and sometimes more. The mulch should be no more than 3-4" thick, spread to the edge of the canopy of trees this size, and it should be kept away from the trunk to prevent disease.

I had a problem like that when I overwaterd my lisbon with miracle grow.