Short history lesson:

The Gulfstreams were originally operated by Executive Jet International/NetJets International/NJI, Inc.

At one time, the division attempted to operate the fleet under an LOA with the FAA for the call sign "NetJets" while the smaller fleets operated by NJA used the call sign "ExecJet."

Because neither call sign is recognized by ATC outside North America, both entities used the "N" number for international operations.

Because of the high number of international operations being conducted by the Gulfstreams, NJI management felt the constant switching between the "NetJets" call sign and the "N" number was too cumbersome. Therefore, the Gulfstream fleet switched to just using the "N" number.

Since integration of NJI and NJA was completed in 2010, management has apparently decided not to rock that particular boat and the Gulfstreams continue to use "N" numbers. To be honest, I'm not sure which protocol the new Globals are using domestically.

Clear as mud?