How can the percentage difference between 68890 and 40756 be 51.3179%? [closed]

It really depends on the formula you use. The formula on the site you used is: $$100\% * \frac{|A-B|}{\frac{A+B}{2}}$$

Basically the absolute difference, divided by the mean of the 2 values. This is one method used to find percentage difference, but I agree it is not intuitive. For example, using this formula going from \$100 to \$200 would be: $100\% * \frac{100}{150}$ or + $66.666\%$. Using Wolfram Alpha's percentage difference calculator we see the formula used there is:

$$100\% * \frac{|A-B|}{A}$$

This formula would give the value you are more likely expecting of + $100\%$ in the previous example, or $-40.84\%$ in your example.