The Short Version
No source in the available evidence presents the statistical analysis, increment series, or anomaly test that this claim treats as established fact. Multiple credible outlets — including the EU observer mission, AFP, and MercoPress — attribute Roberto Sánchez's vote gains during the cited window to the well-documented compositional effect of late-arriving rural and Andean ballot tallies, which is a textbook example of natural demographic variation, not an anomaly inconsistent with it.