You may not be doing anything wrong. A grey card helps set white balance, but it can’t make very different light sources render identically.

If the two shots were lit by sources with different spectral characteristics—especially things like fluorescent lighting—white balance alone may not fully correct the scene. The eyedropper can neutralize the card, but other colors in the room may still shift differently under each light source, so walls and stones can look warmer or cooler between images.

In other words, a grey card corrects overall color balance; it does not create a complete color profile for every object under every type of light.

If you need a closer match across unlike lighting, a color target such as a ColorChecker Passport can help more, because it profiles multiple colors rather than relying on a single neutral patch.

So the likely issue is not the card or your Lightroom sampling technique, but the fact that the lighting itself is different enough that white balance alone can’t make both photos look exactly the same.