Why do we speak, for example, of a 'common or garden' bicycle, meaning one that simply does the job of a bicycle without alloy wheels, Sir Bradley Wiggins pedals or any other bells and whistles.
'Common or garden' means much the same thing as 'bog standard' and can apply to anything animate or inanimate.
The OED entry says.
d. passing into adj., in the slang phr. common or garden, a jocular substitute for ‘common’, ‘ordinary’.
[1657 W. Coles Adam in Eden xxix. 59 But the Common or Garden Nightshade is not dangerous.]
1892 Autobiog. Eng. Gamekeeper (J. Wilkins) 67 It was as large as a common—or garden—hen.
1896 Daily News 16 Oct. 3/4 Such common or garden proceedings not being to the taste of Noa.
1897 Westm. Gaz. 4 Aug. 8/2, I have—to make use of a common or garden expression—been ‘rushed’ in this matter.