For some reason ic2 stores insulated cable variants in NBT, rather than in the items metadata or as a different item, however the diamond pipe doesn't filter by NBT - instead you can use a list, and click the P button to make it compare items precisely.
We don't filter by NBT by default because NBT is normally used for things with large variance - like written books, or the amount of liquid in a container, etc. For ic2 however it might make sense to compare the specific NBT tags that ic2 uses, as insulated cables should probably be sortable by default. (However that would need to go in compat somehow rather than in base BC).
Thank you for the reference to a list. It was hard to find any info about the list and its usage with diamond pipes.
If you don't mind, I want to try to fix the issue. My idea is to add some registry of comparison rules to the StackUtils. Other mods, like BuildCraftCompat, can use this registry to add their own rules. These rules can represent some "common sense comparison", like the difference between insulated and simple cables.