Form followed function also.
The route sign (2), marker lamps (2, 1 each) and exp/loc sign (1 or the other) each used 600 VDC current for power. The (numbers) signify the number of bulbs. This added up to 5 - the number required to cut 600 volts down to 120 volts, the standard for light bulb manufacturers (and house current).
The EXP/LOC sign was on an either/or switch. The Markers, Route Sign, and EXP/LOC were on one master 600V switch and fuse.
The destination sign worked off the battery, as did the deck lamps. This was so that if trolley power was lost, the deck lamps would remain lit. The IRT retained their kerosene markers for that purpose also. Sealed beams, when they came into vogue, also used the battery, since they were really an adaption of automotive technology.
Streetcars ofter had similar setups, with four interior lamps in series with either a bonnet lamp or headlamp on an either/or switch. In Boston they used battery and trolley powered lamps in the same fixture on a relay, the battery would light if the trolley power was lost. P&W 200-cars had the same setup; quite common in the traction world.