The singleton scope (of JSR-330 etc.) is also modeled here as the "bootstrap" scope, i.e. if an object enters the DI world from elsewhere, then it is of course a singleton, and so all other scopes, being such objects, are governed by the singleton scope. The singleton scope is the one and only scope that is governed by itself.
The idea of the dependent scope in CDI or the "per lookup" scope in
HK2 is really just the absence of a scope. It means that nothing is
in charge of managing any lifecycle so you always get a new thing.
Consequently here the dependent scope is modeled as a lack of scope,
i.e. null.
The dependent scope itself in some notional sense is governed by the singleton scope. This only matters during bootstrapping operations.