Booklet Printing Pages vs Sheets

I recently tried booklet printing.  I asked to print only pages 1-4, double-sided, on 8.5" X 14" (legal) paper.  Since I was only printing 4 pages, I expected only 1 sheet, but the booklet sheets area showed 5 sheets.  When I printed, I got 18 pages printed on 5 sheets.  Why did I not just get 1 sheet with my 4 pages?  I am using Adobe Acrobat Reader DC, 2015 Release, Version 2015.020.20042.  Thanks.