Pager elements not respecting total items - offset

If you add a view where one display has offset in the pager and shows m elements; if you add an attachment to that view showing the offset quantity of rows; and if rows quantity is (n*m + offset) elements; you get a pager with one more item than expected. This extra item redirect to last useful pager item.

Example:

View:
Display: Attachment
Shows: 1 element
Offset: 0
Display: Page
Shows: Full pager (6 elements)
Offset: 1

Total Elements: 19

The expected behavior is to get 3 pages; because (19-1)/6 = 3; but it's getting 4 pages because it's not substracting the offset.