I'm looking at IID Si vs IID USC versions. Table 3, page 418, line 29. It's indeed strange, as for other materials the conversion factors are different. For example, 3" relates to 75 mm. (line 26), 1.5" converts to 38 mm (line 27), 2.5" to 65 mm (line 30), etc.
I think this may be a typo. I suggest you forward this to the applicable Code committee for consideration, this is likely an error that sneaked into the Code books.
If you do, you may also point them that 2.5" sometimes converts to 64 mm. (same page, line 32), sometimes to 65 mm. (same page, line 30). This seems inconsistent.