My tires are 700x23/25C. What does 23/25 mean?
Some manufacturers (e.g., Specialize) offer two width measurements (e.g., 700x23/25c) to indicate that the carcass is 25 mm wide, but that the tread patch is equivalent to a 23 mm tire.  I believe this is done to create a more supple tire (larger volume tires can be run at a lower pressure) while reducing rolling resistance.

Another possibility could be to indicate width/height profile differences. A tire profile that is perfectly found tire will be the same in both. I don't know if any manufacturers are doing this.