The functional behavior of "round" in the "quantizer" function does not work the same in MATLAB 2006a (7.2) as in MATLAB 7.1 (R14). RTL Simulations do not match the FixedPoint Simulations.
Beginning with MATLAB 2006a, MathWorks changed the functional behavior of "round" for the "quantizer" function. The behavior is now implemented using "nearest." Thus, wherever the string "round" was used in MATLAB's 7.1 (R14) release, you have to use "nearest" in the 2006a release.
A fix is coming in the 9.1 Release that will convert the Quantizer Directives that use "round' to use "nearest." Until then you must replace "round" with "nearest" manually if using MATLAB 2006a, or stay with MATLAB 7.1(R14).