How can the activation key be changed?

From the Colour Dog menu in the top left corner of the screen, select the Preferences… command to change the key that activates the Colour Dog magnifier.

Why do the measured colour values sometimes deviate slightly?

When displayed on screen, colour data undergoes several conversions between the different colour spaces of the source, the software used and the screen. Rounding and clipping errors can occur thus leading to deviating values.

Depending on how industry standards are interpreted and implemented by the software manufacturers, there can be, for example, deviations in colour reproduction when using different browsers.

You can eliminate one source of error by setting the colour profile of your screen to sRGB. Note, however, that with this setting your display may have difficulty rendering all colours in the sRGB colour space one hundred percent.

Here is the corresponding article in Apple’s user manual:
Change your Mac display’s colour profile