If it pulls as it should and idle is fine, I'd say it's normal. They really all do that!
If your slides are not in sync (if they are way out of sync I mean), it might be a carb sync issue (way out of sync carbs will also cause backfiring - but probably not just off throttle).
But IMO if it has the power it should have, and the idle isn't hunting too badly neither hanging, then it's normal - the off throttle backfiring. Also it's a better practice to ride these bikes instead of worrying about every hickup it throws at us / repairing (what isn't really broken) it constantly:)
2005.5 950 Adventure (with 265mm travel suspension