Diaphragm control valve face to face length

[7016]

Hi All,
Can anyone tell why does a Diaphragm control valve doesn't follow any standard for face to face dimension (i.e., body length)?

 

[7016]

further to my above doubt, please find the attachment of the valve picture for easy understanding.

 

[skengg]

you can search on google about diaphragm valve in which you'll find others manufactures catalog which show face to face dimensions & standard.