This is because according to VMware documentation, this is the ideal design of VMware Cloud Foundation domains that the architect should recommend based on the Bill of Material for different vSAN ready nodes. This design can provide optimal performance and availability by using vSAN ready nodes with different disk group configurations for different domains based on their workload characteristics and requirements. The management domain can use vSAN ready nodes with one disk group per node to provide sufficient capacity and redundancy for management components such as vCenter Server and NSX Manager. The workload domains can use vSAN ready nodes with two disk groups per node to provide higher performance and throughput for VI workloads such as VMs and containers.
Based on VCF Domain Requirement and vSAN Cluster Practice, 4x Nodes will be a Managemnet Domain (Min. Requirement 4 Nodes) other 4x Nodes (with 1 Disk Group) and 8x Nodes (with 2 Disk Groups) seperate 2x Workload Domain because vSAN Best Pratices same cluster should be same numebr disk group and same number of Cache and Capacity (same size) each host .