Short answer
"Worldwide" and "US Only" are two separate job feeds, not one feed with a filter. US clients' jobs appear in both — so if you target US clients through the Worldwide feed, you're competing against freelancers from everywhere and missing the lower-competition US-Only pool.
How to choose
Want US clients? Set the campaign's region to US Only. It's a smaller, less crowded feed of US-based clients — usually higher value and easier to win.
Want global reach? Use Worldwide — but remember it includes US jobs too, alongside everyone else's.
Want both? Run two campaigns — one US Only and one Worldwide — so each is tuned to its feed.
Common gotcha
Naming a campaign "US Only" doesn't make it US-only. Check the actual Region filter in the campaign's Jobs tab — if it's set to Worldwide, you're targeting every country no matter what the campaign is called.