I have a meter that does something similar. A light comes on when it determines the PH is stable. Don't trust the auto stable detection. sometimes the probe may take a long time to fully stabililze. But that said the difference between 6.9 and 7.5 is not enough to worry about.
But if your probe is not agreeing with the API test it could mean the API test is wrong or your PH probe is out of calibration or has fiailed. You can get buy PH calibration reference liquid and then use them with the manufactures instructions to calibrate the probe. And you could take a small sample of the PH calibration liquid and test it with the API test kit. It should tell you which test is reading correctly.
Either buy calibration fluid to find out if it works. Or take it to a LFS that uses digital meters for reading pH and test yours against the store's.
As the probes get old, it takes longer for the meter to stop changing. They need to be calibrated once a year and most reefers calibrate every 2 months when used on a controller.
Yeah, I calibrated this thing as soon as I got it. It came with all the pre-mixed calibration fluids. 4, 7, and 10.01. In fact, I calibrated it twice because after the first calibration, it was reading the pH fluids at about .2 above what they were supposed to be (4.2, 7.2, 10.2). After the second calibration, it seems to zero in on the calibration fluid pH pretty quickly. For instance, if I put it in the 7pH fluid, the happy face will come on around 6.9 or 7.1pH. Then, about a second later, it'll level out at 7.0 and stay there.
In my aquarium water, however, it shows the happy face in a typical amount of time, but then after a good minute or so, it's crept up about .5~.7 pH.
Either way, I have a support ticket in with the company to see if this is just typical of their meters. For now, I'm going with the assumption that it's more correct when it shows the happy face and not when it levels off since that seems to be closer to what the API results are also telling me. And the API fluid claim they don't expire for another couple years.