Accuracy of floating point calculations

[Twilight]

D calculation:

   writefln("%12.2F",log(1-0.9999)/log(1-(1-0.6)^^20));

837675572.38

C++ calculation:

   cout<<setprecision(12)<< (log(1-0.9999)/log(1-pow(1-0.6,20))) 
<<'\n';

837675573.587

As a second data point, changing 0.9999 to 0.75 yields 
126082736.96 (Dlang) vs 126082737.142 (C++).

The discrepancy stood out as I was ultimately taking the ceil of 
the results and noticed an off by one anomaly. Testing with 
octave, www.desmos.com/scientific, and libreoffice(calc) gave 
results consistent with the C++ result. Is the dlang calculation 
within the error bound of what double precision should yield?