As a WP you have it pegged; on many flights QF will block the seat beside you - it shows as 'taken' but is indeed empty. On really lightly loaded flights they will block rows!

Earlier this year I was a bit surprised when I found myself in Row 7 Aisle on a 734 with WHY from row 4 - tried to get more forward several times during the day (at least to the converta seats in rows 4/5) with no result.

On boarding I found I was the most forward WHY PAX of an aircraft < ¼ full; I was easily able to reallocate to row 5 after the seat belt sign went off.