You're talking about the 777-200ER and 777-200LR, right? ER and LR are just two ways of saying the same thing. The original 777s were the 777-200A and 777-200B, when built they became the 777-200 and 777-200ER. When Boeing came to designate the 777X, they could use the 777-300ER because they hadn't already used it, but had to come up with something else for the 777-200X and so they decided to call it the 777-200LR.
Embraer also did this with the ERJ-145, there is the ERJ-145ER and the longer range ERJ-145LR - maybe Boeing copied them ?!?!?!?
Hope that helped