Actually i think it's correct! Look carefully at your surface around the top edge. As you examine the top edge, you can see the surface direction is same as what's happening in the Offset. It's just being amplified in the Offset, so you notice it then. I think the original path curve is curved back at the end a little bit. You might try moving or deleting the second to last point in that curve to reduce the bend in that spot before sweeping...

jonah