We have created a help text box in python that draws text to 3D viewport when our tools run modal. This help box automatically wraps the text if it is too wide to fit in the space given. Our line breaking code uses blf.dimensions(0,text) to determine the width of the text.
On Linux and Windows machines, this works as expected, but on OSX machine it is off by factor of ~1.2. For example, blf.dimensions(0,text) gives 155.4 on OSX and 186.0 on Linux when text = 'Contours Help (click to hide help):'. Note: it should report 186.0 for both machines.
We have seen this issue since at least Jan 12, but thought it was a bug in our code. We are using the default font at size = 12 with DPI = context.user_preferences.system.dpi, which is 72 on all our test machines (Linux, Windows, OSX).