It's not just the population difference.
Other studies show much different stats: http://sdi.sagepub.com/content/22/4/437.extract
The 19th-century death-tolls alone seem comparable to 20-th century.
http://necrometrics.com/wars19c.htm
However, the simplest metric is death per pop.
Study said 44.4 per thousand
Yet with roughly 100 million deaths to 10 billion pop. gives an easy calculation ( https://www.quora.com/How-many-people-lived-during-the-20th-century ), about 10 per thousand, less than previous time periods.
(That is the 20th century, WWI and WWII. We are already into 100 years since the start of WWI, and will be passing Nov 11, 2018 in 2.5 years.
Why is it unrealistic to look at the past 70 years as a metric?)
With respect to the major discrepancy between 44.4 deaths per thousand, and 10 or so deaths per thousand - WT stated in its 2011 YB to ‘TRACING ALL THINGS WITH ACCURACY’
"In summary, the Writing Department insists on using only material that is accurate and truthful, even regarding seemingly insignificant details. As a result, “the faithful and discreet slave” can consistently supply spiritual food that brings honor to “the God of truth,” Jehovah."