Some strings were wired in series. Some were wired in parallel at line voltage. This is because it was possible to make bulbs at just about any voltage from 0.5 to 500. The lower voltage tends to be more reliable.
LEDs, however, are not so easily done. There is only small range they can make the voltage drop (they are a fixed voltage drop and need a ballast to prevent a cascade current much like fluorescent). There is a limit to wiring them in series. And wiring them in parallel requires a ballast on each bulb since the power source had to provide current for all. So these are often made in groups where it is series within the group, and the groups are in parallel on the string. This can lead to extra wires along the string. if the string has 3 or 4 conductors going along, this is probably what it is.
I may try LED strings this year (and maybe tear one down to see how poorly it is made).
