Clusters are competing together on fees. They advertise fees twice a day to the network (thus to all dApps).
dApps, in their configuration (the CAS - Cluster Allocation Strategy, ) can select the cluster with the smallest fee at any point in time.
Stable fees are provided by healthy competition among clusters, possibly leading to segregation on specific use cases not perturbing other use cases in case of congestion (such as CryptoKitties).