More cores are going to help with background (OS) tasks and indeed since SU5 some elements (glass cockpits) were given a core.
I think that the main benefit you are seeing might actually be the small instructions per cycle increase that the 9900k has over the older chip.
Even though their base clocks and turbo are the same the 9900k is capable of processing more per cycle.
Glad you are seeing better performance though.