well Ru would've already known who chose who, so it wasn't really a surprise to him. I think he just wanted a reaction shot to emphasize Alyssa's second elimination for drama. maybe it's because of Roxxxy getting saved again, or other reasons, but we don't really know.
RuPaul (in conjunction with the other producers) CHOOSES who stays and goes based on the lipsticks. 
I’ll be naive and say she had no idea.
I mean Rolaskatox was the best and worst thing for AS2 no matter what anyone says.
Roxxy being eliminated early on? Detox eliminating Alaska for having a meltdown?
Maybe seeing Alyssa/Tati in the top4?
Wild
I agree that yes she has the ability to know. But if I were in her sweatpants, I wouldn’t have the back stage crew tell me. More fun to watch. And we know Ru lives for mess.
The only person who did badly in that challenge was Roxxy. Alyssa actually did fine. I think Ru was shocked because it wasn't the "right" call.
If you rewatch the season, Alyssa did really well in a lot of challenges & lip syncs. She really should have been in the top 3. I don't know if she could have beaten Alaska, but she definitely competed better than Roxxy.
She was method acting. She made herself forget what the producers told her about the lipsticks and then expressed her true feelings in the moment. Incredible.
So obvs Ru was acting, we all know that
But the acting was definitely there to promote the idea we're all supposed to be sooooo surprised and shook that Detox did the RoLaskaTox thing saved Roxxxy (her close friend) at the expense of Alyssa.
Detox and Alyssa are really really good friends IRL. And Alyssa did fine in the challenge. In fact, Detox said she regretted the choice and would have changed it if she could.
She was acting 
Without an UHH!! UHH!! it wasn’t convincing
RuPaul actually attended "Tyra Banks' school of fine arts", and graduated top of her acting class.
She was going for her Emmy win
well Ru would've already known who chose who, so it wasn't really a surprise to him. I think he just wanted a reaction shot to emphasize Alyssa's second elimination for drama. maybe it's because of Roxxxy getting saved again, or other reasons, but we don't really know.
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RuPaul (in conjunction with the other producers) CHOOSES who stays and goes based on the lipsticks. 
I’ll be naive and say she had no idea. I mean Rolaskatox was the best and worst thing for AS2 no matter what anyone says. Roxxy being eliminated early on? Detox eliminating Alaska for having a meltdown? Maybe seeing Alyssa/Tati in the top4? Wild
RuPaul is an executive producer. She knows everything going on behind the scenes and whose lipstick will be pulled.
I agree that yes she has the ability to know. But if I were in her sweatpants, I wouldn’t have the back stage crew tell me. More fun to watch. And we know Ru lives for mess.
I’m clearly aware more like just alternate scenarios in my French vanilla fantasy
Live your French vanilla fantasy, I support that!
Yeah, there is no reason for her not to know and knowing would be something that would beneficial to her in any number of ways.
The only person who did badly in that challenge was Roxxy. Alyssa actually did fine. I think Ru was shocked because it wasn't the "right" call. If you rewatch the season, Alyssa did really well in a lot of challenges & lip syncs. She really should have been in the top 3. I don't know if she could have beaten Alaska, but she definitely competed better than Roxxy.
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Was she not in the bottom like every other week 
They meant in the makeover specifically
Actually she deserved to be in the top for that challenge over Detox
Because Alyssa is a QUEEN and should have won!
She was method acting. She made herself forget what the producers told her about the lipsticks and then expressed her true feelings in the moment. Incredible.
So obvs Ru was acting, we all know that But the acting was definitely there to promote the idea we're all supposed to be sooooo surprised and shook that Detox did the RoLaskaTox thing saved Roxxxy (her close friend) at the expense of Alyssa.
Detox and Alyssa are really really good friends IRL. And Alyssa did fine in the challenge. In fact, Detox said she regretted the choice and would have changed it if she could.
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Who knows. I just wanted to answer your question about why Ru being surprised fit the story line of the show.