The problem with HID is that they'll shift in their spectral output as the internal electrodes and components degrade or migrate into the silica or quartz tube. That's why you're seeing such a broad difference. You also might be seeing MH versus MV lamps, MV lamps are a deeper color blue whereas MH tends to be more white.

If you want a supplement light, I would suggest getting LED panels. They'll last a LONG time as flowering supplements, they won't shift nearly as badly as HID lamps will, the yield improvement for lower power is wonderful, and you can REALLY push the potential of your HPS. Even though I run pure LED, if I had the space, I'd run a combo LED/HID system. Maybe next gen the LEDs will be better than HPS in every aspect, but not quite yet!