Quick test: change "600" to "bold" and then to "900". Make sure it works as intended all ways. 600 is an extremely awkward number: normal weight may be either 400 or 500 depending on what's going on in the font; bold starts at 700.
Yeah I found the answer elsewhere. Arial only goes by 'normal' or 'bold' and the browser chooses whatever it is closest to when a number is used. Not sure what changed overnight that 600 was previously closest to 'bold' and now is closest to 'normal'.