Try using a rod or a stick of some sort to stir is and see what you get. That's odd. Honestly I've tried it all sorts of ways and it's exactly the same no matter how I do it, and it's within 200 PPM of the readout on the Universal 40... So.. it must be right? If I see cheap strips somewhere I might try those now too. You have me curious. You do know (it's not a secret) that I do keep a second non-Taylor test kit and compare the two against each other periodically to cross check because I don't just trust one kit. I guess I learned this when doing fancy water chemistry when beer brewing. Now I basically use RO and formulas I came up with and don't bother testing for that....
One thing I do find sort of critical is the very first drop. It seems to take a second or two to become cloudy. Maybe you need to wait longer between drops?
And a video with phosphate remover for a background... here?

It's odd because I thought the strips I would get from DSP would have been salt strips in the accessory kit, but no.. they were phosphate... I maybe should try one.. haven't even opened them up....