The problem with DATEDIFF is that it's sensitive to temporal borders. For example, the following two times are only 3 milliseconds apart but the DATEDIFF for hours between the two returns a "1".

SELECT DATEDIFF(hh,'12:59:59.997', '13:00');

An easy correction for this "feature" is to simply convert the times to DATETIME, subtract the smaller from the larger, and then do a DATEDIFF between 0 and that previously calculated difference to get the number of whole hours that have passed (without rounding).

SELECT DATEDIFF(hh,0,CAST('13:00' AS DATETIME)-CAST('12:59:59.997' AS DATETIME))