Why does my Instance Variable behave like global?

It doesn't — it's per-instance. You're confusing them with Global Variables. Instance Vars store per-object; Global Vars are project-wide.

Player health: Instance Var (each player has own). Game score: Global Var (shared). Inventory: depends on whether per-player or shared.

Family Instance Variables apply to all family members. Useful for shared mechanic across object types.

Fix: Construct 3 Instance Variable Acts Global

Quick answer: Instance Variables = per-instance. Global Variables = project-wide. Pick by what should be shared.

Player Instance Var Score increments. Display shows total across all players. You picked Instance when you wanted Global.

The Fix

For per-player score:
  Player object → Instance Variable: Score (Number)
  Each player has its own.

For total team score:
  Project → New Global Variable: TeamScore
  All events read/write this single.

For shared across object types:
  Family → Instance Variable: Faction
  All family members carry it.

Pick by intent. Convert by deleting and recreating in the right scope.

Verifying

Per-player score updates only the affected player. Team score updates project-wide.

“Instance per-object. Global project-wide.”

Related Issues

For Array picking, see array. For Tap double, see tap.

Right scope. Vars behave.