accuracy of rest kits

So..I have been testing my cal level using sera and been getting readings 380 to 420 ppm. But since my test kit solution ran out, I got myself an API... Now the readings on this seems to tell my cal is between 500 to 520ppm. Not sure what's going on... Both test kits expiry dates are good. Did test on API twice and got the same result. .. Anyone experienced this ??