Not telling...
But seriously, short of posting the exact formula here those calculations are hard to explain except in the general terms that the above suggests already.
You can Dual Wield, the chance of successfully making a strike with that dualed weapon depends on your level of skill in Dual Wield and is affected by your dexterity and luck scores.
Being a lucky character who has Grandmaster'd Dual Wield and is blessed with a high dexterity will mean that that chance is higher than the norm.
Truthfully, I'm not even 100% certain that the new combat code makes such explicit use of these stats as previously.