Yeah I found that interesting as well. I've never seen the lighter ipe, only the very dark stuff, and have only made one ipe bow so far, which I promptly gave away to a guy who needed a decent hunting bow.
I've heard that ipe is actually a number of different species so I can imagine it may vary from one species to another. Although I've also thought that the light colores ipe sam harper shows in that page looks alot like cumaru, and I wonder if it isn't cumaru, because I know (personally) cumaru can/will fret more easily. Food for thought. I wouldn't be suprised if Cumaru is accidentally or wrongfully sold as ipe because its almost just as hard, just as dense, and much less well known, and its as least as cheap as ipe.