Wrong expectation for floating point numbers -- short story is if a fraction is not exactly representable by 1/2^n, it'll not be able to be stored exactly and so what you get is the closest representation that can be stored.
All the skinny is in <Goldberg, What Every Computer Scientist...>>
If you want only n digits of precision past the decimal point, set that in the format string.
I don't know anything about Simulink; can't you pass the actual variable into it, not a string? Then you get the closest binary representation without any rounding, whatever the value. All it's going to do with the string is convert it back to internal representation, anyway.
