Every time a trade occurs between $WBTC and $ETH using this pool, the pool-specific swap fee plus the Swarm fees is taken from the trade. The pool-specific swap fee goes back into the pool and is added to its assets, ever increasing its market cap. If previously there were 100 SPT tokens representing 100 ETH and 100 wBTC each token would be worth 1 ETH & 1 wBTC. If one user now traded 10 ETH for 10 WBTC, and another traded 10 WBTC for 10 ETH, then there would now be the 100 ETH, plus 100 WBTC, plus trading fees for two trades. This means the prorated amount that SPT represents has now increased by the same amount when it is withdrawn.