Looks pretty random, and I'd understand if it were using characters that may exhibit special properties (such as excessive height like an ```f``` which may be taller than the line); but as it is there I have no idea why.
Besides, its only used in the lines afterwards; and then again overwritten by the call to ```dc.GetTextExtent("W", &w &h);``` 3 lines onwards.
Unless it does something fancy thats not in the documentation, it could probably be removed altogether and just modify ```rowHeight``` after the 2nd call.