As a software developer, I completely expect this; it's just an everyday
consequence of limited-precision math. You'll get the same kind of anomalies in
almost any programming language, and other kinds of calculations can produce far
worse inaccuracies than this (seems kind of odd to single out standard
deviations which are relatively well-behaved, but maybe Excel used to have a
worse algorithm). John Gustafson proposed using clusters of unums and then
posits [http://loyc.net/2019/unum-posits.html] to mitigate the problem
(preferably in hardware), but it's not practical to eliminate it entirely.

I tried a set of 104,857,600 values in Excel 2010 for Windows and the standard deviations diverge at the 9th decimal:

Count 1: 104,857,600

Count 2: 104,857,600

Sum 1: 5,242,888,545

Sum 2: 104,862,842,888,545

Average 1: 50.00008149

Average 2: 1,000,050.00008149

Stdev.p 1: 29.15276094096330000

Stdev.p 2: 29.15276094160490000