Rated vs nominal volume
The nominal size on the sticker ("50-gallon") is a marketing class; the rated storage volume in the certification record is the actual measured capacity, typically 5–10% less (a "50-gallon" tank often holds ~45–47 gallons).
Insulation, internal heat exchangers, and manufacturing variation eat into the sticker number. Certification records use the measured figure, which is why our pages may show 45 gallons for a tank marketed as a 50.
Sizing decisions should use first-hour rating anyway — two "50-gallon" tanks can differ by 20+ gallons in first-hour delivery.