Why The Odd Ball Design Of The Oveta Culp Hobby Stamp Pane?

The recently issued C. Alfred Chief Anderson pane (#15 in a series of distinguished Americans) follows a similar design of its predecessor (#14 Jose Ferrer), which brings me to question why the Oveta Culp Hobby pane has such an odd-ball design. This pane has no header and no indication that it is the #13 in the same series.

Beginning with the Margaret Chase Smith stamp, the USPS started to put a header and indicate the number on the distinguished Americans panes. So, if one collects the panes in this series, the Hobby pane sticks out like a sore thumb. Does anyone know the reason behind this anomaly?

I've come across many articles that mention this fact, but none provides an explanation.