Could be due to
-webkit-font-smoothing, there are 3 possible values:
none– turn font smoothing off, show the text with jagged sharp edges.
antialiased– smooth the font on the level of the pixel, as opposed to the subpixel. Switching from subpixel rendering to antialiasing for light text on dark backgrounds makes it look lighter.
subpixel-antialiased– on most non-retina displays this will give the sharpest text.
Usually the second one does the trick of returning font to normal weight.