Varying the lead length on the LED packages eases identification of the pins. It also serves to ease insertion into PCBs. The longest lead is located in the board first, followed by the next longest lead, etc., gradually advancing the component into the board until all are located and the LED pushed in to the required depth.
Note that many of the packages have shoulders on the leads to provide a positive stop against the PCB for ease of alignment, etc.
T-sizes
Early LEDs were made to conform to standard “T” sizes. The “T” designation indicated tubular shape and the numbers after indicated the tubular diameter in 1/8″ multiples. The original LEDs were made in T1-3/4 or T1.75 (5.5 mm – 5 mm nominal) and later in T1 (1/8″ = 3 mm) packages.