1800 RPM is the standard speed for rating them. (a 1725 RPM motor is an 1800 RPM motor by design. the nameplate says 1725 because in reality, that's what you actually get with magnetic "slip" factored in. But there is zero difference between two motors labeled 1725 or 1800 RPM.)
So, to be less confusing, 1800 is the "standard" speed of a motor. A 3450 RPM (3600 by design) motor is considered to be running at twice "standard speed".