Certificate matter: Which certificate has the highest C/P ratio?

A question often raised by customer: why the same grading of a diamond, different certificates have different price? Usually diamond with GIA certificate is more expensive than that of IGI / HRD. Does it mean GIA is better then the other labs?

An organization took 10 diamonds to different labs for grading. Detail as below:

http://www.diamonds.net/News/NewsItem.aspx?ArticleID=43417

Lower the score, more strict of the lab.

GIA           14.8

IGI            14.9

EGL USA  15.4

HRD         16.3

EGL HK    19.5

EGL Israel 19.8

Is it more strict of the lab, the diamond with the same grading is more expensive?

The total value of the 10 diamonds, compare with the market price, are as below:

GIA            $34,703

HRD           $33,868

EGL USA   $31,686

EGL Israel  $29,032

IGI             $26,623

EGL HK    $24,367

What conclusion can you draw?

Note:

1.     Both GIA and IGI have several labs over the world. Why only EGL seperates into USA, Israel and HK lab?

2.     This study includes only 10 diamonds. The sample size is small compare with tens of thousands of diamond in the market.

3.     The organization has its own labs not included in the study.

4.     HK has several local diamond labs gain HOKLAS from Hong Kong Accreditation Service (HKAS), whose not include in the study. If they are included, it would increase consumer confident about local diamond labs.