EDIT/UPDATE: SOLUTION

I took the advice of installing DisplayCal and followed this tutorial - and the results were MUCH better! At first I wasn't particularly impressed with the results, but frustration pushed me to try out a xyz lut and a 3 hour calibration cycle instead of the suggested single curve and 15 minutes - and that hit the mark. I examined the resulting tone curve, and lo and behold, my display has a 'notch' around 50% grey (128,128,128) were the red green and blue colors differ much more than on the rest of the curve. I also noticed that manually fiddling with the red, green and blue display settings revealed an issue where adjusting the green response between 93% and 94% affected the red and blue quite a lot - which does not happen on my other display. I am convinced that I have a slightly wonky unit with some minor errors, but I won't pursue that any further now.

Original post:

I have two displays, both Dell U2717D's attached with DisplayPorts. AlthoughI bought them a few months apart they are identical. I also have the Spyder5elite which I use to calibrate them. Problem is, I don't find the calibration very precise. I calibrated both monitors individually to start with and then the StudioMatch function to match them to each other - and the result is not very convicing (to my eyes at least). My standard windows background is just a neutral grey (128,128,128) on both screens and it makes it quite obvious that one monitor is more greenish than the other (or the other is more reddish). I can manually match them by eye using the RGB sliders on my display, but I wouldn't think that to be necessary. Quite disappointed with this product unless something else is affecting the colors.

Example with proper white balance. (My ceiling lights are 5500 K temperature, verified): https://imgur.com/TVYQsoO

Does anyone else have any experience with this problem? Is it just the precision you can expect?