Long ago in science class we learned that deep within every living organism are cells. The starch cells in potatoes remain somewhat intact, and perform how we like them when cooking. However, potatoes (any type or variety) can actually be over-mashed. During normal cooking, the potato cell molecules swell and separate. The process of starch-grain bonding is called gelatinization. This can vary slightly in complexity, depending on the size of starch molecules within varying potato types, and potatoes typically turn out how we expect when following a recipe. When over-mashed however, the cell molecules break down internally one step too many, the molecule cell structure further weaken, and the result is the gluey, pasty-ness you describe.  

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