Other time periods will obviously given you other numbers. If you back out the Apache codesets, which LO gets merely via their status as a downstream consumer of AOO, the relative contribution of the top 18 goes even higher. (Since we're talking about community trends, not code, it makes no sense to count code contributions that did not originate within the community. Otherwise we'd count every author of every 3rd party library we use, right?)
So however you slice it, the "long tail" may be long, but it is very, very thin. This is the way it has always been with OpenOffice, from the earliest days. Most of the development work is done by professionals, and the community mainly works on translation and marketing. LO has not really changed this fundamentally. The "long tail" is just a myth they tell their SUSE and RedHat executives to convince them to continue pouring in cash to prop up the project. But with declining PC sales, and the niche Linux desktop market, I really don't see that continuing for much longer.