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@gbj_tester
In the olden days of metal steerer tubes, it didn't matter that the stem clamp was slightly above the top of the steerer - in fact, that was the recommended set up. However, this does cause a highly concentrated crushing force on the steerer, because the upper clamp bolt has extra leverage, and carbon steerers are not actually very good at resisting compression. By effectively adding a 5-10mm ring of carbon above the minimum necessary to just fully enter the stem clamp, a substantial reinforcing effect is achieved for a weight penalty of just a couple of grams, much less than would be added by bonding a metal reinforcing tube inside the steerer. As a side benefit, by not having the stem overhanging the end of the steerer, and thus bending round it and forming a conical interface, the propensity of stems to "walk" up the steerer during cyclical loading, and thus loosen the headset preload, is all but eliminated. I've been running a spacer above the stem on all my forks since I sat down and thought about the design for a while, and on my best bike I have a custom top cap which incorporates the spacer
