different wall distance recommendation - Bose 901

[skyfly]

Any insights? Especially, the Japanese recommendation is very different.

the distance from the rear of the speaker to the wall behind the speaker

Bose 901 Series VI: 20 - 45 cm
Bose 901 Life style: > 20 cm, 40 cm
Bose 901 WestBorough: 20 - 60 cm