ISM Saddle Rails

Installing a new ISM PS 1.1 saddle this evening to replace the Selle Italia Nekkar Plus that was on my bike when I got it. When I went to put my rail clamps on the ISM, I noticed that the pitch between the rails isn’t exactly the same. On my Selle Italia saddle, the clamps line up perfectly with the rails. That same clamp on the ISM is very slightly too wide. I measured and the ISM rails appear to ~1mm closer together than those on the SI saddle. I can force the clamp down on the ISM but it visibly tweaks the saddle slightly. Am I missing something? Are there really saddle clamps out there that are 1mm different in pitch? Should I just clamp it down and not worry about it?