Does either test kit come with a calibration solution? I personally only use test kit brands that come with them. Seachem, as cheap as it is, comes with calibration solution and I always found it to test spot on. I now use AquaForest and they also come with calibration vials. Get yourself one of these and then test the calibration sample with the other two test kits and see how they all fall.
Getting 470 and 480 on two back to back readings is actually really good. Testing water parameters is pretty damn complicated. The kits in this hobby make it look easy, but you really have to have everything exact if you want to reproduce results.
First off, our sample sizes are way too small. Did you fill the vial correctly with sample water? Was the sample at the bottom of the line, the middle or the top? The level of the meniscus makes a difference when dealing with such small samples.
You can skew the test results by getting a drop size wrong at any step. Were the dropper bottles perfectly vertical? When filling syringes, was all the air purged?
Was all the equipment rinsed after the last test with RO/DI water? If not, you could be including residuals from the last sample, which will increase the test results.
Was electronic testing equipment calibrated at each test?
These are just four examples on why reproducing test results is so hard. This is also the reason professional water testing is expensive. In the hobby, we use way too small water samples and don't follow any of the lab procedures used in industry....but expect the same precision and accuracy on the test kits....not possible.
Let the corals guide you, they know what they are doing
