Hi again,
it seems you have a general issue with your system and/or handling of your PMDG addon, because also here - I can´t reproduce it - the same as in your report here (KSEA 16L ILS issue)
Here a few screenshots in the same configuration - no weather, night:
1000f … two red, two white
500ft … two red, two white - exactly on the glideslope:
So, Hervé is absolutely correct with his answer. It´s first the position of your seat and second the differences between PAPI and GS antenna.
Compare the altitude with the charts, and you see that this is exactly the same. You see on the PFD 440 feet, on the chart you see the TCH displaced threshold 56’. So 440 - 56 = 384 feet … exactly the runway height.
At threshold, according my PFD in the PMDG 737 - at 440 feet:
Hope that helps,
Richard