The default graph

The SPARQL documentation says:

Operations may specify graphs to be modified, or they may rely on a default graph for that operation. […] The unnamed graph for the store will be the default graph for any operations on that store. Depending on implementation, the unnamed graph may refer to a separate graph, a graph describing the named graphs, a representation of a union of other graphs, etc.

Tracker defines the default graph to be the union of the unnamed graph and all known named graphs. Updates without specified graph are still performed only on the unnamed graph.