I was a Orianna vs Lux in game, later in the game i noticed the big notice in ap beetween me and lux with very similir items. She had 200 ap more. so i did the math she had 20 less ap than me in items. And yes we both had deathcap, SO HOW can she have 200 more ap than me ? bug?
It may surprise you to learn, but some people actually take scaling AP runes.
You can dissect the match pretty well with Riots new match history on the website. http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1757982970/43454715?tab=builds&participant=6
We can see here that she was using AP masteries where you were using AD. She had +12 AP and +5% AP bonus just from that. I don't know when she made Mejai's, but if it had a lot of stacks that adds up fast too.
Edit: You can click a specific player on the Select a Player bar to see their specific masteries/runes/item build path
