Why do all general counsel think their law department is unique?

At the obvious level, they are right. Every person being unique, every law department, a fortiori, is unique.

That no-argument point accepted, the felt differences have much less consequence when you look at a department in terms of its processes, structure, culture, software, and other attributes. Then the department shares many traits with other law departments, even more so with those in the same industry and size, and the uniqueness shrinks to little. In management terms, each law department certainly has its quirks, which is to be expected, but overall many departments of roughly the same size, let alone industry, face and resolve roughly the same management issues.