Why would one twin excel in a sport yet the other be quite the opposite considering they are genetically identical in every way?

it goes back to the nature vs nurture debate. How much of what we do is based on genetics and how much is based on events that happen to us. They may be genetically similar but they no doubt have different personalities and interest.

If they both have the same of everything is it still not possible for the non sports twin to achieve the same standard if he/she so chose.

Environment has some say. If a care provider neglects to care for one twin while helping the other, then it makes sense that he/she will have a one-up. Genetics determine the range, but environment and other acting influences are factors in this matter.

it would be, but its hard to say whats holding the other twin back. thats where social sciences run into problems.

You are, I take it, only talking about identical twins. Fraternal twins are not genetically identical.

If the former, it is a matter of focus and desire.

Genetically the potential is there for the other twin to excel, however for one to excel at anything there is the added necessity of that rare component: practice!
Without it, all the genes in China won’t amount to didly!

My brothers are identical twins. One excelled in music, getting a full scholarship to college. The other went to college by excelling at football. Both had the same opportunities. Both took music lessons and sports. They just chose different paths in highschool. Their wives are total opposite personality types. As adults their lives are so different. Nothing will ever change their feeling of closeness that only twins share.