How does AD decided when to enable SyncOnStartup for inbound replication partners

I have been playing with the MSAD_ReplNeighbor wmi class for monitoring AD replication.

I have a hub and spoke, single domain, topology where three DCs are in the spoke site.

One holds all the master roles and the other 2 DCs hold no master roles.

I ran a query with the MSAD_ReplNeighbor class on each of the three hub DCs and I am curious about what I'm seeing with the SyncOnStartup property:

On DC1 (which holds all master roles) the SyncOnStartup bit is set for the Domain and Configuration partitions, for all SourceDsas (execpt the ones in the same site). SyncOnStartup is turned off for DomainDnsZones and ForestDnsZones
On DC2 (holds no master roles) the SyncOnStartup bit is set for the ForestDnsZones partition, for all SourceDsas, and is turned off for all other partitions
On DC3 (holds no master roles) the SyncOnStartup bit is set for the DomainDnsZones partition, for all SourceDsas, and is turned off for all other partitions

Of course the SyncOnStartup bit is set for all SourceDsas in the DC's same site

So my question is how did AD decide to load balance like this (which is what I assume what its doing).

What would it do if I introduced a fourth DC into the hub site?

I'd just like to understand this better.