When a weapon NFT is MINTED OR TRANSCENDED, the scaling, critical chance, and critical damage are randomly rolled on a scale determined by its rarity.
Each weapon type has its own scale.
When a weapon is improved within its rarity, they are increased by X%.
When a weapon is transcended to its new rarity its starting stats are rolled new on the appropriate scale.
So even if you were unlucky and got bad scaling as a common. When transcended it may still become a top scaling weapon.
Since you can not transcend an Eldritch weapon anymore, consider carefully if you want to transcend a Mythic weapon, especially if its a very powerfull one at the top of its range.
Tip: use the Marketplace to look at possible scaling for your weapon.
Further clarification:This means that an excellent or masterpiece Rare weapon with all improvements may have better numbers than the masterpiece Mythic weapon without improvements it becomes after transcending.
But after a certain number of improvements, the mythical weapon becomes stronger again.