It all has to do with what went before...
...or rather, the specified procedural codes assigned by ATC.
For example, when transponders were first used in the USA, they had only two digits that could be changed.
The last two were fixed.... zero, zero.
During this period, three airway structures were in use also, low (below 14,000, Victor airways), intermediate (14,000 - 23,500), and high (FL240-FL450, Jet routes). The positive control area started at FL240.
Flights planned for final cruising altitudes below FL240 were assigned a departure squawk of 1100.
Flights planned for final cruising altitudes at FL240 or above were assigned a departure squawk of 2100.
For flights that left the continental control area, with a final cruising altitude of less than FL240, were to squawk 1000, when entering oceanic airspace.
For flights that left the continental control area, with a final cruising altitude of FL240 or above, were to squawk 2000, when entering oceanic airspace.
For those flights that remained in domestic airspace (either enroute to oceanic airspace or not), the departure squawk was changed to an enroute squawk at the first crossed ARTCC boundry...and at every boundry thereafter.