Some tires aren't as they seem. Some, for example, say they are 700c x 25mm, but when you measure the diameter of the tire, it may measure 28mm when properly inflated. Other brands might be more true to the numbers.

If you measure the tire diameter with calipers, compare with the chart and input that size rather than the tires indicated, it may improve the accuracy of the cyclometer you're using.

BikeCalc.com - How to calculate Bicycle Wheel Size

Or do what the above post read, either way, you'll get a ball park answer...