My bike uses a tire size of 700x23/25C. I understand that the 700 indicates the diameter of the rim. 23 is supposed to be the width of the tread which I believe means 23 mm, am I right? So what does the 25c 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.