Earotic Motown [[No 1)

This was a 2009 CD with a pink cover and a picture of a naked woman holding a saxophone. Essentially another excuse to release same old in a Various Artists package, but they noted the "different" versions where appropriate. I always find this sort of thing fascinating but generally can't usually tell where the differences are when I listen unless they're really obvious. Presumably, where they don't specify anything it'll be a standard stereo album cut.

Anyhow The Originals track is shown as "single reference mix" and I've often wondered what this really means - anyone have a simple explanation?

007-531-948-6
Earotic Motown
Various Artists
Motown / Universal Music Group GmbH
2009

Tracklist
01 Marvin Gaye & Tammi Terrell - Ain't No Mountain High Enough [Ben Human Remix] 3:25
02 Diana Ross & The Supremes - My World Is Empty Without You 2:33
03 The Commodores - Easy 4:16
04 Michael Jackson - Ain't No Sunshine 4:13
05 Four Tops - Reach Out I'll Be There 2:59
06 The Isley Brothers - This Old Heart Of Mine [[Is Weak For You) [single version - mono] 2:46
07 The Temptations - My Girl [single version - mono] 2:59
08 Brenda Holloway - My World Is Crumbling 3:09
09 The Contours - I Like Everything About You 2:53
10 The Originals - Why When Love Is Gone [single reference mix] 2:52
11 Lionel Richie - All Night Long [[All Night) [album version] 6:18
12 Martha Reevs & The Vandellas - [[Love Is Like a) Heat Wave [extended stereo mix]
13 Smokey Robinson & The Miracles - I Second That Emotion [single version - mono]
14 Stevie Wonder - For Once In My Life 2:51
15 Tammi Terrell - All I Do Is Think About You 3:00
16 The Jackson Five - I'll Be There 3:57


Cover here
https://www.flickr.com/photos/cookep...v4dwx-2jv2NT1/

Could it have been considered as a single release and then rejected. The "Green Grow The Lilacs" version is stereo and 2.47. and "Cellarful of Motown" is mono and 2.53. Was there a mono version of "GGTL" ?. By the way I love the cover!