What the article actually says is that increasing the severity of the filter has more effect on the number of health sites blocked than on the number of porn sites.
That's not the same thing as saying that they're more likely to block health sites.
Here's a completely hypothetical example:
Total number of health sites: 100
Total number of porn sites: 100
Number of health sites blocked with less restrictive filtering: 1
Number of porn sites blocked with less restrictive filtering: 90
Number of health sites blocked with more restrictive filtering: 50
Number of porn sites blocked with more restrictive filtering: 92
In this example, increasing the severity has little effect on the number of porn sites blocked (increasing from 90% to 92%, which is not much change) but has a big effect on the number of health sites blocked (increasing from 1% to 50%, which is a big change).