[cvoss]

The radiation on the way in has a different frequency than on the way out. For example, there is UV included in sunlight. But black body heat at Earth's temperature radiates in infrared. Clouds are very opaque to infrared, and more (though not completely) transparent to UV.

[jcims]

This is the actual answer. The clouds aren't differentially reflecting sunlight into and out of the atmosphere, they are reflecting the black body radiation emitted by the earth.