It's true, not all tree shade is created equal. Broad-leaved, deciduous trees, in general, cast a broader shadow than many evergreens, such as pine species. This is due, in part, to the shape of the mature tree canopy. When grown in the open, deciduous trees tend to have a decurrent growth habit, which results in broad, round shade patterns. Pines and most other evergreen trees have an excurrent growth habit, which results in more pyramidal or columnar, narrower shade patterns.
Additionally, most deciduous trees do not have lower limbs that might block a desirable cool breeze. Finally, broad-leaved trees evapotranspirate more during the day, due to their higher leaf area. This can lead to a cooler environment near the tree.