The historical sources for Minard's graphic (listed in his paragraph at the top in the final version of Napoleon's March, as shown on page 176 of The Visual Display of Quantitative Information and on the poster) describe the enormous chaos during the entire disastrous invasion. On this, see Segur's book, Napoleon's Invasion of Russia, one of Minard's sources. It is clear that no one is taking a daily census of the Grand Army; the death numbers are surpassingly fuzzy.
By the way, Minard counts only the Grand Army. With Russian casualties, the total number of deaths is estimated at 700,000.
To reason about variation of death counts, it is informative to recall the difficulties in obtaining an accurate account of the numbers of deaths in the attack on the World Trade Center. During the two months following the attack, the death count went down from the initial estimate of 6,000 to the final confirmed total of 3,000.
Now contemplate how much greater measurement errors would be in 1812 in a chaotic disaster spread all over western Russia with some 250 times the total number of deaths compared with the WTC attack.
Wobbles in Minard's numbers are to be expected.
-- Edward Tufte