>That would seem to yield 5,613,179 articles by February 28, 2018, still a notable discrepancy to the official number on [[Special:Statistics]] (5,600,831 right now, a month later), but a smaller one.

A  <1% difference is not surprising. Please see: https://wikitech.wikimedia.org/wiki/Analytics/AQS/Wikistats_2/Data_Quality

We have vetted this data (per project) with wikistats1 metrics definitions and in most cases differences are less than 1.5%. Again, per project. We expect this to shift a little as we continue working on data quality but we will always see small differences with other data sources.  Some variability comes from data definitions, some for sources and computations.