Apart from implementing it incorrect, an uncalibrated display could also cause this. Check out http://www.lagom.nl/lcd-test/gamma_calibration.php with DPI scaling turned off, at 100% zoom level (how browsers scale images are also horrible, so you want to avoid that).

edit:

Reading back, viewing the gradients also not at 100% zoom level could also itself cause the mismatch, because browsers just suck at image scaling.