Profiling question

I use a Dell 30" Color Critical monitor and have been very satisfied with it so far.  It is approximately 4 years old.  For the first couple of years when I would profile it monthly I could not tell a difference between the "before" and "after" shots after the profile was completed, but for the past two years or so I can tell a pretty good difference.  I use an X-Rite ColorMunki Display (Ver. 1.1.4, which is the latest version) to profile it.  I always reset the monitor to factory default settings before profiling and then begin.  I set the luminance to 100.

Is it common for a monitor and/or puck to begin to "swaying" from it's factory setting over a period of years.  It came from the factory fully calibrated and that calibration matched X-Rite's perfectly at first.  How can I know for sure which instrument to believe, the monitor or X-Rite?

Images below:  To be honest, I prefer the "before" images, but certainly don't want to use that setting if it's not accurate.  The difference isn't staggering, but the "After" images are definitely warmer with a slight yellowish cast to them.

It's probably the monitor, they do shift over time and as they get closer to the end of their life they can shift dramatically. the thing to trend over time is the profiled dE - or the deviation from perfect colors. If suddenly the number starts going up and you can no longer achieve a decent dE (which I define to be less than 1 and I define a great dE to be less than 0.5) then it is time to start thinking about a new monitor.

As for the puck, other than making sure the lens is nice and clean, I have never found them to drift even over more than a decade. Also a change in graphics card or even a change in graphics card driver could change the before and after look.

So overall, seeing it change from before and after isn't a problem. it's only when the monitor can no longer be calibrated to proper colors does it become concerning.

Hope that helps.

Oh, the dE is something that the calibration software should spit out at the end of every calibration.

Thanks E.J. Great answer - lots of solid info. I had never even considered it might be graphics card related, and I need to upgrade my card anyway because it is only 2 GB.