Why does array loses it internal capacity on length change?

[Uldis via Digitalmars-d-learn]

While writing a structs function that I wanted to minimize 
allocations and use an internal preallocated buffer, but I 
noticed that arrays are losing their capacity when its length is 
changed.

For example:

void main() {

         int[] a;
         a.reserve = 1024;

         void dump(in ref int[] b,size_t line = __LINE__) {
                 import std.stdio;
                 writeln(line, ": Buffer length = ", b.length, " 
capacity= ", b.capacity);
         }
         dump(a); // line 10
         a.length = 0;
         dump(a);
         a ~= [1,2,3];
         dump(a);
         a.length = 0;
         dump(a);
         a ~= [4,5,6];
         dump(a);
}

gives output:

10: Buffer length = 0 capacity= 2043
12: Buffer length = 0 capacity= 2043
14: Buffer length = 3 capacity= 2043
16: Buffer length = 0 capacity= 0
18: Buffer length = 3 capacity= 3

but I expected:

10: Buffer length = 0 capacity= 2043
12: Buffer length = 0 capacity= 2043
14: Buffer length = 3 capacity= 2043
16: Buffer length = 0 capacity= 2043
18: Buffer length = 3 capacity= 2043


Why is this happening, how to avoid it?