Most business today would be well pleased with their loss/Theft ration being that low. Especially when you concider the number of slaves in border states like Missouri, Kentucky, Maryland and even Texas and Virginia/West Virginia, where runaway slaves were far closer to the free states and Mexico than say in Mississippi, Alabama of Georgia.
To explain that low of an accumulated grand total over a period of ten years, you have to concider either an awfully effective slave patrol system throughout the south, and therefore a lack of oppertunity for the slave to runaway. Or maybe a lack of desire on the part of the majority of the slaves to runaway in the first place. I know that it would be difficult for me to "quit" my job if I didn't have some form of backup plan and didn't know where my next meal would be coming from or who my friends would be in hostile territory, and knowing that I would be probably punished in some way if I were caught. But then again that all would be far easier to organize in the border states with supposed freedom so near at hand.
And yes I know that last comment is really contrivercial in todays political charged atmosphere. But then again 1/4 of 1% over a ten years period is a remarkable stop loss program record for any business.