A Google search yielded a possible answer shown in the below link, which is a transcription of the instructions to enumerators for 1860. This is from the University of Minnesota's Integrated Public Use Microdata Series (IPUMS).

The 100 in Column 6 likely refers to the number of slaves that had run away and not been returned to that owner. Here's the text you'll find:

6. Fugitives.-- Under heading 6 insert, in figures, opposite the name of the owner, a mark or number designating the fugitives who, having escaped within the year, have not been returned to their owners. Such fugitives are to be described as fully as if in possession of their masters. No allusion is to be made respecting such as may have absconded subsequent to the 1st day of June; they are to be recorded as if in possession of their proper owners.

The number is supposed to next to the owner, not a slave, so I don't know whether this answers your question or not.

However, is the female slave listed on the same line as the owner, as it is for the 1860 Slave Schedule I found for the person who owned my family? If so, the 100 does indeed refer to the owner and not to that particular slave.

Hope this helps.