In IT everything consists of Bits and Bytes, where a Bit can usually have one of two possible states, either zero ore one (on or off), thus base-2 (or Binary), and one Byte consists of 8 Bits.. The marketing guys however like to use base-10 (dezimal), which makes their systems' capacity look larger than it technically is. Take a look at e.g. http://en.wikipedia.org/wiki/Megabyte which explains the difference between binary and decimal values.
With 2TB as in your case, if this was a base-10 value then your LUN would only be ~1.81TB (base-2) and you would be save. However, it looks like the storage system works with base-2 values, i.e. real 2TB.
André