Most 20-sided dice have the decency to arrange the numbers so that opposite faces always add to 21. But they do not make sure the numbers are evenly distributed around each vertex.
This "numerically balanced" d20 has six vertices which have numbers around them summing to 52, and six summing to 53. This means there is no 'high side' or 'low side' to the dice and makes the results rolled more evenly distributed.
If you want the numerically fairest d20 on the market: this is it.
Price = p101 pence = £5.47
As seen in Matt's video about Dice Stacking!