rand() doesn't do what you think it does. It's a multiply then an add, then return particular bits of the current seed. See "Linear congruential generator" for more information.

On GCC, it's multiply by 1103515245 then add 12345, and return low 30 bits. On MSVC, it's multiply by 214013 and add 2531011 then return bits 16...30.

I don't specifically mean stdlib `rand` (hence comment about assuming no inherent bias) - but just some arbitrary PRNG. Updated the code above to say `random` so as not confuse.