I figured it had something to do with different ages. At some level it still felt like if the micromort-based estimate of lifespan is 124 years, then someone group should have a life expectancy that long. But based on your comments, maybe the real issue is that, say, a baby that dies on the first day experiences 1 million micromorts per day, while someone who lives to be 100 experiences (1 million)/(365.25 days per year x 100 years) = 27 micromorts per day.
Wait, something still doesn't make sense, because even at age 100, 27 per day on average is more than the 24 per day that we get from using deaths in a year.
But the population is actually skewing older (see below) as it grows, I think in part because much of the population increase is coming from immigration and increased life expectancy. Source: https://www.census.gov/prod/cen2010/briefs/c2010br-03.pdf