Page size after pdfcrop don't match my calculation

I run

$ pdfcrop --margins '-36 -59 -49 -35'  tmp.pdf tmpo3.pdf

But the page sizes before and after don't match with my calculation. My calculation shows that the page sizes in points after cropping should be 612-36-49 = 527 by 792-59-35 = 698 pts. But pdfinfo says it is 471 x 670 pts. I wonder why I am wrong?

My pdfcrop is PDFCROP 1.20, 2009/10/06 - Copyright (c) 2002-2009 by Heiko Oberdiek.

$ pdfinfo tmp.pdf 
Creator:        Preview
Producer:       Mac OS X 10.7.5 Quartz PDFContext
CreationDate:   Wed Sep 18 07:45:10 2013
ModDate:        Wed Sep 18 07:45:10 2013
Tagged:         no
Form:           none
Pages:          6
Encrypted:      no
Page size:      612 x 792 pts (letter)
Page rot:       0
File size:      85737 bytes
Optimized:      no
PDF version:    1.3

$ pdfinfo tmpo3.pdf 
Creator:        TeX
Producer:       pdfTeX-1.40.10
CreationDate:   Sat Nov 29 08:49:30 2014
ModDate:        Sat Nov 29 08:49:30 2014
Tagged:         no
Form:           none
Pages:          6
Encrypted:      no
Page size:      471 x 670 pts
Page rot:       0
File size:      251449 bytes
Optimized:      no
PDF version:    1.4