This is a sensitivity from the config file %USERPROFILE%\Documents\My Games\Rainbow Six - Siege\[USERID]\GameSettings.ini, it'a not available in-game. It's more accurate to set the sensitivity in the config file, which is why it's the default selection.
If you don't want to set it in the config file however, you can select In-game instead, but this assumes that the values in the config file are unmodified, i.e. that MouseSensitivityMultiplierUnit and ADSMouseMultiplierUnit are both set to 0.02.
1.3 in Apex converts to about 5 to R6 (there's just a -0.1672% discrepancy) when matching the 360 distance, there's is absolutely no question about this.
https://www.mouse-sensitivity.com/?share=ed296b432ba70a9204e96b98a547fc86
If you are not getting the same 360 distance it's most likely because of one of these reasons:
Your mouse driver is loading a different DPI in one of the games.
The multiplier values in the R6 config file are not set to their default values.
Raw input in R6 can in some cases be bugged, turning it off might actually make the sensitivity more accurate.