What’s the subject for the verb suppose?
Now, yer mum an’ dad were as good a witch an’ wizard as I ever knew. Head boy an’ girl at Hogwarts in their day! Suppose the myst’ry is why You-Know-Who never tried to get ‘em on his side before. . . probably knew they were too close ter Dumbledore ter want anythin’ ter do with the Dark Side.