You haven't stated any numbers showing that the intensity of solar radiation at the surface is lower than at the top of the atmosphere. Your data only show that the value at the top of the atmosphere is different on different dates.
From our vast experience, however, we do know that the solar intensity at the surface IS lower than it is at the top of the atmosphere, simply because the atmosphere absorbs some solar radiation ... different amounts of it at different wavelengths.
That's the main reason, for example, why the sky is red at sunrise and sunset and blue the rest of the day, and why the temperature of the air is so much higher than 3° absolute, and why we aren't broiled by X-rays all day. Also the reason why it's worth the tremendous cost and makes such a difference to build astronomical observatories on mountain tops and in low-Earth-orbit, instead of in convenient deep valleys.
