Light levels affecting IV curves for a Diode
Reading up on photodiodes will quite probably help.  

Wereyour reference diodes in their original optically opaque outer package?
 Are your new diodes "bare"  / light exposed or protected from light?



Wikipedia says this re photodiodes.

Probably relevant is:


Photoconductive mode: In this mode the diode is often reverse biased (with the cathode positive), dramatically reducing the response time at the expense of increased noise. This increases the width of the depletion layer, which decreases the junction's capacitance resulting in faster response times.   

The reverse bias induces only a small amount of current (known as saturation or back current) along its direction while the photocurrent remains virtually the same. For a given spectral distribution, the photocurrent is linearly proportional to the illuminance (and to the irradiance).[4]  

Although this mode is faster {than photovoltaic mode}, the photoconductive mode tends to exhibit more electronic noise.[citation needed] The leakage current of a good PIN diode is so low (&lt;1 nA) that the Johnson–Nyquist noise of the load resistance in a typical circuit often dominates.




Probably relevant:



Graph below from here.