The avg lifespan is 10 years. Since there are 5 born per decade, 5 die per decade. The population just didn't pop up and start all in phase, it contains animals of all ages.
If there are five antelopes born and five antelopes dying per decade, the average lifespan is not ten years--it is two years.
Consider: in a million years (number chosen large so as to render phasing effects trivial) there will have been 12,000,000,000 antelopes that lived, 24,000 at a time, a total of 24,000,000,000 antelope-years. It doesn't matter whether some animals lived for millenia while others lasted only minutes. The total number of antelope-years accummulated will be 24,000,000,000 (since a herd of 24,000 antelope will accumullate 24,000 antelope-years per year) and the total number of antelope will be 12,000,000,000 (birth rate of 5 antelope per antelope per decade). Thus an average of two antelope-years per antelope, or (more simply), two years.