Plz help me 2 understand?

I was having the exact same problems as "Heroesvoice", only diff is I'd already encountered the MacScan solution and ran it (total of 6x's now). Result on the very 1st try DNSChanger found and properly isolated. Foolishly I did not discard to the trash until after the subsequent 2nd running of MacScan (in full authorized mode btw). I dumped isolated item from desktop once 2nd running "found" item on desktop...ok; so I'm admittedly slow. Runs 3-5 were clean (run over a 9-day interval). Ran 6th scan, found 1 tracking cookie; so total of 2finds for the year of iMac usage...I'm thrilled.
Now the problem: the whole time from the 1st scan, I was still being redirected as if nothing had ever been found and isolated; out of extreme exacerbation I googled my prob and found this forum. I followed the separate link/suggestion for the DNSChanger Removal Tool as well as all other suggestions...AND AM VERY GRATEFUL -->it worked. But why? Why did I have to use the removal tool separate from MacScan? I can attest that my MacScan updates are (and have been) current.; it makes no sense 2me. Can someone plz help me to understand what-happened?
Also; my PC friends for the past 10yrs or so are all massively and aggressively anti-virus/anti-spyware experts...they've had 2b. I'm not! This Mac is my very first computer ever. I'd expected that I was "immune" to this sort of thing by purchasing Mac. Can someone advise plz...I'm concerned; though not scared.