You nailed it. See the KM Wiki Variables
Global Variables
Global Variables are permanently stored on your Mac drive (like a file), and are available for read/write in any Macro or Action, not just the Macro where the Variable was created. These variables persist between logins and restarts of your Mac.
TL;DRTFM
Basically all variables are "global" variables, UNLESS the Variable Name starts with one of these:
• Local
• Instance
OR, Start OR End With:
• PW
• Password
Does that make it clear?