Here is the replay from last night. I was in the middle of typing when you replied. I have it cued up to just before Monica & Val were cut. (If people want to see the whole elimination just click on the video & view it directly on
YouTube and rewind to the beginning.)
3 more couples are saved. Tyra isn't just reading the names. She's saying stuff in between, "The next safe couple is. . .Also dancing next week are. . ." So she was still following some scripted stuff in between the names.
Then Tyra announces that Anne & Keo and Vernon & Peta are the bottom two. So they should be on stage at that moment. Which they were. Chrishell & Gleb were an extra couple on stage.

I'm wondering if the Chrishell & Gleb card was accidentally left out, or was stuck to the back of another card, as she was reading off the names?
Tyra, rightfully asked for clarification. As the production control room was scrambling to figure out what was going on and they are under a time crunch, maybe the poor person who was in Tyra's ear prompter was saying, "There should be another card! Look for another card with names on it!" Meaning the one with Chrishell & Gleb's name on it.
Tyra had already announced that Anne & Keo and Vernon & Peta are the bottom two. She probably should have just asked, "Are Chrishell & Gleb safe?"
Maybe, in the pressure of the moment and needing to come up with an instant answer,

when the person in her ear said, there should be another card with names on it, OR, just pick any other couple to go with Anne & Keo, (but meaning the two remaining,) Tyra randomly opened the stack, looked at whatever names were on that card and said, "The bottom two couples are Anne & Keo. . . and Monica & Val," instead of one of the remaining couples on stage. And suddenly Monica & Val were of the bottom two?
It's odd that when their names were announced earlier, that no one in the control room was going, "Wait! Aren't they one of the bottom two couples?" And that two other couples who were supposed to be safe, were still on stage, also not caught earlier.
Bruno was a trouper, mentioning how mistakes like this happen on the DWTS UK version, too. It's part of live TV. And Derek mentioned that he thought this is like what happened on the Academy Awards.