Recently I decided to try a different brand of tyre - just because... they were marked 29x2.25 (ETRTO 57-622) and hours of reading reviews I gave it a go.
I mounted these tyres on a 25mm ID rim which is 2mm wider than the rim used by the manufacturer's reference measurements. The tyres were no where near the expected width of 2.25 inch or 57mm, they measured 53mm which is the width of a 2.1 tyre. Don't get me wrong, 4mm is not a lot but visually on a tyre you will be amazed at the difference in volume and appearance.
I started measuring the new tyre after taking it off the rim and found that the width from bead to bead was 138mm compared to the 150mm of all the other 2.25 tyres (ETRTO 57-622) I could get my hands on. Taking the formula for the circumference of a circle (c = 2*pi*r) and adding the width of the rim to the width of the tyre it was clear that the 12mm difference was the reason for the loss of width and volume.
This story is to make you aware of the fact that regardless of the numbers printed in the rubber you still may end up with a tyre that is not what you expected. When trying a different tyre, measure the width from bead to bead (across the tread) before buying.
