Schedulers != CPUs.
You have 8 CPU cores, that means 8 visible online schedulers (one for each core). The hidden schedulers are for background tasks or the DAC, they're created in addition to the one scheduler per core. They're not your 'missing' cores.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability