The site provides actual sizes of what? All the user does is key in some generic sizing info into that website and it makes some rough calculations based on those entries. It doesn't know the actual dimensions of any tires, otherwise it would require the user to pick specific brands and models of tires in addition to entering the generic p-metric sizing, and the width of rims they will be installed on.
The actual tire you buy in whatever size may vary from brand to brand and model to model within a brand. The dimensions will also vary depending on what width rim they are installed on. It's theoretically possible that you could find a 255/55R20 and a 255/50R20, or a 255/50R20 and a 265/50R20, tire that actually measure out to the same circumference.
Some sites provide more detailed info on tires so you can get an idea of the circumference variance among tires that are marked as the same size. For example, in 255/55R20 size tires, the measured circumference could be as little as ~93" to as much as ~96" which would give a diameter of ~29.5-30.5". This roughly translates into the smallest tire requiring ~680 revolutions per mile whereas the largest one would only require ~660. At the end of the day, you really need to install the actual tires on your rims, inflate them, and measure them to confirm actual size.
For those that ascribe to the school of thought that AWD systems are very picky and require closely matched tires, then you should always measure your tires whenever replacing them, to make sure they are within whatever tolerance level you are comfortable with.
Tires 101 on Tire Matching on All Wheel and Four Wheel Drive :: Souza's Tire Service
ETA - Peter, you may want to measure the actual circumference of your Blizzaks and then measure your 20s and see what they actually are. I believe the Blizzaks fall a little closer to the smaller side of specs (~91" circumference which should be ~29.1" diameter - compared to the 255/50R20s, I imagine they will be around an inch larger diameter and up to 3" more on circumference - depending on wear on both tires). I've had a few different winter tires that were the same way - actual measurements were further off than the calculations indicated.