The text appears bold becaue there is less vertical resolution than horizontal. To be exact, the horizontal text uses built in APIs to draw text, but when rotated we draw it ourselves. At small point sizes these our drawing algorithm
can only use screen pixels and are therefore not sub-pixel accurate. The vertical nature of the text exacebates this due to the previously mentioned vertical resolution difference.

There is nothing much can be done to aviod this. If customer like to get a fix we have create a enhancement request.