Some could be due to camera and/or lighting differences, some may have been refinished at some point., some due to differences in the actual wood used or the stain itself.
Klaus' site shows mostly golden oak but also one pair that is more redish-orange, and another set more towards orange..... http://www.infinity-classics.de/mod...-Delta-Sigma-Epsilon-1988-95-98/index-IRS.htm
I did not in my quick search see any that were piano black but there could have been some maybe?