It seems the problem has todo with font:getWidth and also with extraspacing.
For uneven fonts you can't use it and font:getWidth seems to return wrong values.
Test (same for all löve versions): First string uses a calc. x and y position depending on fontsizes Second string uses a fixed x,y position
Until love 0.7.2, 0.8.0, 0.9.2 everything is correctly rendered. Since love 0.10.x, bitmapfonts are crippled.