All "standard" tank sizes are slightly off from what they actually hold. Usually it's smaller, ocasionally it's larger. A 55 gallon only holds like 52.34 gallons of water. Yours being a couple gallons larger than the listed size is not unusual or abnormal. It holds 32.5 gallons, but it's standard naming size is a 30 long. Generally though, the difference in size is not more than 5 gallons. A standard 38 gallon actually holds 37.7 U.S. gallons, simply by adding 3 inches to the height and making it 20 inches instead of a 30g long's 17 inches.
