Well, it's a product of living in an area (Nevada County, CA) where there is a lot of growing going on. Pretty much everyone I know around here has mites. Half of them lie about it and claim that they don't, but nearly all of them do. I kept getting mites because I was getting local clones that had them. It didn't look like they had mites when I got the clones, but around week 5-6 of flowering they would be noticeable and by week 7-8 I would be vacuuming webs off the tops of my buds. It pains me to admit that, but yep I have had some problems with mites over the years. I would also go long stretches without seeing any, but I noticed that it was because I was growing from seed during those stretches and I wasn't contaminating my grow with strange clones.
So, if you live in an area where there is a lot of pot being grown, it's wise to be especially vigilant about accidental contamination. My mistake was that I was dumb enough to believe people when they handed me some clones and said "oh yeah, they have been treated for mites and they are 100% clean". Take my advice and assume that the opposite is true whenever someone feeds you a bs line like that. Maybe they are telling the truth, but you can't afford to be wrong. Mites are the WORST.