508 pixels/inch = 200 pixels/cm, exactly. The resolution unit in PNG is either the meter, or it is unspecified. 1 inch = 25.4 mm, 25.4 * 2 = 50.8. 508 is close enough to 600, and 600 dpi is a resolution commonly found in desktop printers.

If you've worked with PNG files under Photoshop, you may have noticed that a picture saved with exactly 72 dpi has 72.009 dpi when you close it and open it again. Using an integer multiple of 254 for the dpi prevents such a drift.

It's a nice round metric measurement. 508 dots per inch = 20 dots per mm.