Difference in digits10 between GCC and MSVC

I have the following code:

#include <iostream>
#include <limits>

int main()
{
   std::cout << std::numeric_limits<unsigned long long>::digits10 << std::endl;
   return 0;
}

• GCC 4.4 returns 19
• MS VS 9.0 returns 18

Can someone please explain Why is there a difference between the two? I would have expected such a constant would be the same regardless of the compiler.