A comparison of the HST and radio proper-motion measurements for the masers (left panel) and their corresponding uncertainties (right panel). Left: the measurements show good agreement for the masers with radio measurements from D23 (blue points) but show larger discrepancies for those from P22 (black points). Particularly large discrepancies (≳20 mas yr⁻¹) are found for SiO-16, SiO-21, and SiO-25. Right: the HST proper-motion errors (red points) are lower for many sources, especially for those at larger distances, where less radio astrometry is available (R ≳ 20″). Vertical black lines connect the HST and radio proper-motion errors for the same source. In both panels, IRS 9 is highlighted with a filled star.