The c`tan were described as being tied to the fundamental fabric of the materium itself and as such they were nigh on impossible to destroy. Even the necrons acknowledged this. The best they could accomplish was to shatter the c`tan and divide their essences into thousands of shards.
These shards are powerful but still very much fallible. It`s a safe assumption that every encounter previously with a "c`tan" was actually an encounter with a shard. Given the description of the things this is not difficult to believe.
Nonsense is our Salvation