LEDs can emit higher and lower wavelengths around its centre peak. Many others have documented this (e.g. here http://www.apug.org/forums/viewpost.php?p=1363286)

At least two explanations include:
1. Overpowering them which widens their spectral bandwith
2. Dopant contamination (http://www.apug.org/forums/viewpost.php?p=1363650)