What matters in chimney draw is height and cross sectional area. In the case of nominal 8 x 8 inch (measures 8.5 x 8.5 outside), i.e. 7 x 7 inch inside, the area is 7 x 7 = 49 sq inches. In the case of 8 inch round ID, the area is pi x R^2 = 50.27 sq inches. Essentially the same area, especially when you take off some area for the rounding of the four inside corners. Chimneys of the same height constructed of either clay tile liner will work the same. Nominal 8 x 8 size fits inside standard square chimney block, inside opening 9 x 9 inches, outside slightly less than 16 inches square by one 3/8 inch nominal mortar joint. Aren't all these masonry trade dimensions wonderful!
I suspect round clay tiles are measured in ID because it's then easy to select one the mates with the same size stovepipe, e.g., 6 inch clay thimble mates with 6 inch stovepipe.