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Yes, the reduction is probably to allow for L2 overhead. As the overhead varies per packet, you could allow for worst case or average case. Their reduction looks to be for average case (usually in the range of 10 to 15%), but as such, I don't know why they chose the exact value they did.
BTW, from a QoS perspective, if your CIR is 10 Mbps, QoS would work better if you actually ran the interface at 10 Mbps.
