Player values fluctuate based on ownership. If you have players whom other owners add to their roster, their value will increase and be reflected in your cap number. If other owners drop players you have, your cap number will decrease.

Example: If you purchase a player for £8 million and that player performs well, causing them to be added by multiple teams, that player will increase in value, and you can then sell them for £10 million. As a result, you will have increased your cap by £2 million.