Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Taking mid is a good adavantage for Lux to get xp faster than being accompanied which as a result helps her get items faster and win the game easier....HOWEVER.... getting her 2nd skill (Prismatic Barrier) helps her protect herself from getting too much dmg since she's low on hp....Being a good nuker Lux can achieve high AP + that she can briefly support her team as well as push minions to turrets (even though not hightly recomneded)