Doug Martin's website called The Pencil Pages seemed the obvious place to find the answer. Some pencil leads erase easier than others. Generally, softer leads, such as 2B, erase easier than hard leads like 2H. Most pencils have HB leads which are in the middle for erasability. Here's what Doug says...

"Experience tells me that not all leads erase the same. Pencil lead is made from a mixture of graphite and clay, and as the ratio of these ingredients is varied, so does the erasing characteristic. Also, some leads are made from polymers (a type of plastic) and this gives them different erasing characteristics."