If it's around 1/10, it must be over a long period of time. There have certainly been periods where I've returned _way_ more than that, including things like 'These headphones I bought a year ago have broke, this hard drive I bought 6 months ago has stopped working, AND I'd like to return this expensive item I've just bought, and the only other things I've bought recently are £5-10 books'.
I suspect it's more likely to be done on total purchase _price_ - I spend a fair amount of money on things that are very difficult to counterfeit (video games), so they are obviously never returned, which probably tips the balance back in my favour (the fact that I'm in the EU, and have a statutory right to return (for any reason) may also be a factor).