Why InDesign calls double quotation mark "double quotation marks" (plural)?

[Doe Johns]

This might be somewhat unusual question to ask here, but why InDesign (I use InDesign CS6) refers to each of the left and right double quotation marks as plural? Why it calls each of them "quotations marks"  (plural) and not "quotation mark"? I'm a foreigner and this looks wrong to me. 

Type > Insert Special Character > Quotation Marks:

[AuroraPenticton]

Because “ is literally 2 marks so it's plural and ‘ is only one mark so it's singular.

[Doe Johns]

Well, but

* Why nobody except Adobe call them in such a way? I use English-language Internet about 10 years, and I never saw it before.

* Even InDesign uses the singular form if you press Ctrl+F to open the search window and then select grep!

* As far as I know, double is different from "two". Double is "one".

[Mike Witherell]

A better question might be: Why not call straight ones ditto marks? ☺️

[Dave Creamer of IDEAS]

>>Why not call straight ones ditto marks? 

Just don't call them inch and foot marks! 😁