Why will only certain SATA data cables work?

My customer brought in an iMac that had gone to another repair shop, had the drive replaced because the first one was supposedly defective. It worked for several weeks and then stopped booting. We have determined that the first drive is actually operational when we use one of our straight connector SATA data cables from stock. It will not work with the original Apple SATA data cable though - do drive is recognized at all. I was sure that I just needed a new data cable with the 90 degree bends similar to the Apple able, so I purchased one new at CompUSA and tried it out (up to 3GPS). Nothing. The exact same results as with the Apple original cable. Swap back to my straight cable, boots right up. Can there be a difference in the cables themselves? Everything I read say's no, but there sure is something going on there. I wish the straight cable would fit, but I have to use one with the 90 degree bends. :o(

Daryn