TSQL Convert FLOAT to STRING Truncation/Rounding Issue

SQL Server 2008 R2 Why when converting a FLOAT to a string (Varchar / Nvarchar) using Cast or Convert, does a rounding or truncation occur? Using CAST DECLARE @floatVal FLOAT = 4.76758527755737 SELECT CAST(@floatVal AS VARCHAR(20)) 'VARCHAR', CAST(@floatVal AS NVARCHAR(20)) 'NVARCHAR' Output VARCHAR NVARCHAR 4.76759 4.76759 Using CONVERT: DECLARE @floatVal FLOAT = 4.76758527755737 SELECT CONVERT(VARCHAR(20), @floatVal) 'VARCHAR', CONVERT(NVARCHAR(20), @floatVal) 'NVARCHAR' Output VARCHAR NVARCHAR 4.76759 4.76759 With STR however, there's no truncation/rounding DECLARE @floatVal FLOAT = 4.76758527755737 SELECT LTRIM(STR(@floatVal, 17, 14)) 'STR' Output STR 4.76758527755737 Thanks in advance