That happens on EQ. The program is not sophisticated enough to correctly calculate any shape other than a rectangle, a square, a half triangle or a quarter triangle. It seems to assume that any shape, like a diamond is really a rectangle that needs to have corners cut off. If the quilt designer is more of an artist than a mathematician, she or he may rely on this type of estimating.
If I couldn't easily figure the yardage myself I'd rather have extra than be short. Have been through the panic of not having enough.