Remember that *max* efficiency of the HX1050 is 88% (86% under 110VAC, and 89% under 230VAC), which is under ideal conditions/load.
At half load (535w), your PSU just misses the SILVER mark and performs at 86% efficiency, which is between SILVER and BRONZE.
As the article says, we're essentially arguing semantics. You bought a much larger PSU than you needed, but it's not going to cost you much in terms of inefficiencies, and it's not going to harm anything.
The Corsair HX1050 produced a fairly typical set of results in this section that fell a little short of the 80 Plus Silver requirements (85%, 88% and 85% efficiency at 20%, 50% and 100% DC loading) following the OCC power supply testing methodology. The OCC power supply testing methodology doesn't exactly replicate the rail loadings and a shortfall of one or two percent isn't unusual so it would be unfair to mark the power supply (or any other that produced similar results) down in this section. Power factor levels are rarely if ever a problem in high end power supplies, especially with a 110VAC supply, and the HX1050 easily kept the level above the required 0.9 mark at 50% DC loading.