Easy explanation:
.removeAttr() removes the attribute and .removeProp() removes the property
Hard explanation:
Attributes and properties are not the same thing. Properties may have values that were assigned using attributes, however, once the DOM has been built, properties are not always directly related to an attribute.
A good example is the value attribute. The value attribute never changes even after you type into a text input, while the value property is changed relative to your input.
-- Kevin