Floating point computations differ in different runs of same program

Hi,

I have a C program which does lots of computations in floats and
doubles,
It is processing the same file (same data) but in different runs of
the program (on the same machine) the results vary sometimes by
numeric value 1 (which is after rounding the resulting float number)
Is this expected behavior with floats and doulbe (with lots of
intermediate conversion in between) ?

Since There is only one thread so I guess the result should be same
always.

Any reason/pointers for the above mentioned behavior.

Thanks in advance
Rahul