How to explain two NMR spectra of the (allegedly) same compound with slightly different shifts?
That's a pretty small chemical shift difference.  Keep in mind that chemical shifts depend on temperature, concentration, pH, and other factors.  The concentration difference is the most likely, but I wouldn't rule out T effects.  If you're running it in a different batch of CDCl3 (or even the same batch, but it's aged), the pH could be different as CDCl3 becomes acidic over time.  

If you used identical settings when collecting the spectra (e.g. using a default Proton) then you can measure the signal-to-noise ratio of two identical peaks and get an estimate for how different the concentrations are.  You can also simply test how sensitive your chemical shifts are to concentration by making up a more concentrated sample (or diluting your current sample).  Or try changing the temperature with the VTC and ramp it up to 25.5 or 26.0 C and see how much the chemical shifts change (referencing everything to CHCl3 for consistency).

If your compound were reacting with something, there would be a much more dramatic change in the chemical shifts than 0.07 ppm.