When a music asset goes into trading, the market decides the price. The % on each asset shows the current market value compared to the initial price.
The initial price is the fixed price for each share when the asset was first released on the market. When all shares to the initial price sold, the asset goes into trading.
Let's say the initial price for each share was €2 and the latest sold price on the secondary market is €2.40, the asset will show a value increase of +20%.