Thread:Love Robin/@comment-4708882-20180518225407/@comment-366087-20180518231321

The CW producers still view Supergirl as not fully a part of Arrowverse…
 * "As revealed during The Flash-Supergirl crossover last year, Supergirl hails from a different universe. Are you planning on bringing Supergirl into our universe?"
 * "Not for her entire world, but Kara will be traveling from her dimension to our dimension, “our” being the world that The Flash, Arrow, and Legends lives in."
 * http://www.ew.com/article/2016/10/07/supergirl-season-2-cw-spoilers/


 * "How will Kara Danvers make her way to the Arrowverse for the upcoming four-way crossover between Arrow, The Flash, DC’s Legends of Tomorrow, and Supergirl?"
 * "“Last year, Supergirl established that Flash was able to make his way to what I call Earth-CBS, and it stands to reason that, with the proper breach technology, the reverse can happen,” executive producer Marc Guggenheim told reporters Tuesday following a screening of this week’s Arrow and Legends of Tomorrow."
 * http://www.ew.com/article/2016/10/18/arrow-crossover-100-episode-spoilers/

The producers make sure the franchises jump through a few hoops for the characters to crossover, as well as clearly defining there is a difference in *their minds* that the universes are different.

Short: Supergirl is NOT Arrowverse. It is, at best, "Arrowverse-adjacent".