[Xaccers]

Meaning that if the ban hadn't been introduced 20 extra non smokers would have been brought in with smoking related heart attacks, and only 17 extra smokers would have been brought in with smoking related heart attacks.

Take 200 people, lable half of them smokers, and the other half nonsmokers.
That's how many of those people who would have had smoking related heart attacks if the ban hadn't been brought in.
Now, introducing the ban prevented some of them having having heart attacks.
Take 17 away from the smokers, and 20 away from the non-smokers.
You now have the people from this group who would have had smoke related heart attacks had the ban not been brought in.
Only 17 of those who'd carried on smoking directly, and 20 of those who had only passively smoked.
So a greater proportion of passive smokers would have suffered smoke related heart attacks had the ban not been brought in than the proportion of smokers.
Unless of course the science behind the statistics is actually wrong in order to make it look like the ban was a good thing (duh like dodgy statistics are required for that).