Funny issue with casting double to ulong

[Saurabh Das via Digitalmars-d-learn]

Consider this snippet:

void main()
{
     import std.stdio;
     auto a = 6.2151;
     auto b = a * 10000;
     auto c = cast(ulong)b;
     writeln("a: ", typeof(a).stringof, " ", a);
     writeln("b: ", typeof(b).stringof, " ", b);
     writeln("c: ", typeof(c).stringof, " ", c);

     auto x = 62151.0;
     auto y = cast(ulong)x;
     writeln("x: ", typeof(x).stringof, " ", x);
     writeln("y: ", typeof(y).stringof, " ", y);
}

The output is:
a: double 6.2151
b: double 62151
c: ulong 62150
x: double 62151
y: ulong 62151

Why does c round off from 62151 to 62150 when casting to ulong?

Thanks,
Saurabh