A: You’ve noticed that Grafting is, in fact, “the regular way to win Fluxx” by simply combining two (number) Keepers to make a larger number.
The Grafting Rule, however, is meant to make it possible to create numbers for the specific purpose of subsequently using them in an equation to reach the Goal number via calculation. The card’s exact wording is:
“Two Keepers can now be joined together to form a single two-digit number for the purposes of Victory Rule calculations.” (emphasis added)
So, no, you can’t just graft One and Three together to meet the Goal Unlucky. You could do it if you had an EXTRA One, though:
1 3 x 1 = 13