How accurate are the crystals on ST's Nucleo boards?
I found the bill of materials for this board on the product page:

It has these two descriptions for some crystals:


So it looks like they are probably ±20 ppm. That is actually usually the tolerance over the full specified temperature range -- I'd expect even less variation at room temperature.

However, you did state that the 8MHz crystal is actually not present on this board, so I'm not sure if this is the right crystal. Even if it's not, there's a good chance they use the same one on the programmer.