How to explain two NMR spectra of the (allegedly) same compound with slightly different shifts? For my synthetic lab work I synthesised a Horner-Wadsworth-Emmons phosphonate; for the sake of the question let’s assume it to be the one presented in the image below.



When first synthesising it, I of course recorded NMR data. I performed a Horner-Wadsworth-Emmons reaction using a tiny bit of the phosphonate and kept the rest in the fridge.

One month later I retried the HWE and got very bad yields, so I decided to record another NMR spectra. The two spectra themselves look identical and the signals (those that aren’t multiplets) also show the same coupling constants. However, when referencing the spectrum to the chloroform signal ($7.26~\mathrm{ppm}$), all signals are shifted by the constant value of $\approx 0.07~\mathrm{ppm}$.

I decided to check the old literature spectrum from 1986, but it was unhelpful: It only reported multiplets such that both of my spectra comfortably fall well within the multiplets’ limits.

Considering that the two spectra look identical and that the difference between two signals corresponding to the same hydrogens is identical as are the coupling constants, I want to assume that both spectra show an identical compound. However, the $0.07~\mathrm{ppm}$ shift for everything save chloroform still irritates me.

Both spectra were recorded at $500~\mathrm{MHz}$. The parameters of the earlier one state a temperature of $25~\mathrm{^\circ C}$, the later one was apparantly measured at $25.1~\mathrm{^\circ C}$. However, I don’t think that that could explain the shift.

What could explain this shift difference?

(My first thoughts were some changes to the compound that only affect the phosphorus atom, but aside from deoxygenation being chemically unlikely shouldn’t it also cause random chemical shift differences including slightly different coupling constants? In fact, shouldn’t any chemical modification cause the shifts to change in a seemingly random fashion depending on the proximity of hydrogen in question and entity modified?)

After checking because I was asked in the comments: The later spectrum (the one that has higher shift values) has broadened peaks. That includes the chloroform peak.
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.