When trying to find the voltage between two points on a schematic I get the opposite answer
\$V_{AC}\$ corresponds to

$$
V_{AC} = V_A - V_C = 12 V
$$

You interpreted bad relationship subscripts.

Look at the references in your textbook. Surely, the author has defined how to interpret the notation.

Also, points B and C, are the same pontencial so that

$$
V_A = V_{AB} = V_{AC} = 12\,\mathrm{V}
$$