SHINY
A shiny NFT will always receive the maximum increase when upgraded.
When an item is upgraded (a star filled), that improvement will be determined by RNG from a range. For instance, +3 to +8. A shiny item will always receive +8 in this case.
The 1-star rusty shiny items in the game were the result of a bug. Their shiny status is useless in the game because they cannot be upgraded. They have no second star to fill.
But they may be of interest to collectors.