Floating point values have a limited precision. Hence a value might 
not have the same string representation after any processing. That also
includes writing a floating point value in your script and directly 
printing it without any mathematical operations.

If you would like to know more about "floats" and what IEEE
754 is, read this:
http://docs.sun.com/source/806-3568/ncg_goldberg.html
 
Thank you for your interest in PHP.

1.003 is stored as slightly less than that (close as possible), which can be seen when you show enough digits:

printf('%.20f', 1.003);

Gives 1.00299999999999989164 for me...