Most people don't know very much about investing. Lacking knowledge of their own and not knowing where to turn for advice or instruction, they may read a few magazine articles or try to wade through an investment-advice book to try to learn more. And while many of those books are filled with good intentions, they're often overflowing with bad advice.
Among the worst forms of bad advice is the nonsense that all risk can be quantified and contained equally. This belief comes through in many forms, like the one that says nobody should have more than 10% of their assets in a single stock, or that one should have bonds in his or her portfolio equal as a share of the total portfolio to the person's age (20% bonds at age 20, 50% at age 50, and so on). These might be simple rules that can form a starting point, but they are hardly good rules for universal application. What's risky depends on what you know.
To offer an extreme example, had Warren Buffett followed the advice about holding bonds equal to his age, then he would have left 35% of his wealth in bonds at the time he took over Berkshire Hathaway. He probably wouldn't have lost money on that 35% -- it would have been very safe -- but it would most certainly not have been less risky invested there than it would have been in his own company, which by one measure of increase is worth 4,340 times what it was at the time of the takeover. Buffett knew better than the rule of thumb.
Buffett, of course, is an outlier in the world of investing. But he's an excellent illustration of the case that a person with exceptional knowledge and judgment should in fact bet heavily when the odds are strongly in their favor.