Hi Lu,
By default, MATLAB stores all numeric values as double-precision floating point. The fact is, almost all decimal numbers are NOT representable exactly in floating point arithmetic as a double. This is why this issue is completely about precision and the binary form that a double comprises.
You can read in detail about this here: http://stackoverflow.com/questions/10904132/conversion-of-numeric-to-string-in-matlab
If you are interested in converting a double or single input to the exact decimal string, you could refer the following file exchange submission: https://www.mathworks.com/matlabcentral/fileexchange/22239-num2strexact--exact-version-of-num2str-
Hope the above suggestions help!
