Complete removal creates more of a carbohydrate overage at the wound site. Phloem transports carbs from parts of the plant that are creating them. Here's a quote from a bonsai site:

"Removing the bark, cambium, and phloem prevents carbohydrates and photosynthates from flowing down the trunk. It still allows water and mineral nutrients to flow upward to the leaves via the channels in the xylem, thus keeping the leafy portions of the tree from drying out and maintaining them with an adequate supply of nutrients. Removing the cambium layer prevents the regeneration of phloem and healing over of the wound, the carbohydrates and photosynthates flow down the trunk and collect at the girdling site. The presence of this excess of carbohydrates and photosynthates at the girdling site, plus the presence of the water in the sphagnum moss, causes dormant adventitious buds in the area to grow into roots."