Wrong pointer calculation without casting on struct

[tcak via Digitalmars-d-learn]

On Friday, 6 February 2015 at 03:59:51 UTC, tcak wrote:
> I am on 64-bit Linux.
>
> I defined a struct that it 8 bytes in total.
>
> align(1) struct MessageBase{
> align(1):
> 	ushort qc;
> 	ushort wc;
> 	ushort id;
> 	ushort contentLength;
> 	void[0] content;
> }
>
>
> I defined a function in this struct that tries to set a pointer 
> to "contentLength" field.
>
> writeln( "Without: ", (&this + id.offsetof) );
> writeln( "With   : ", (cast(size_t)&this + id.offsetof) );
>
> Results:
> Without: 7FFFF74F5030
> With   : 140737342558228
>
> 0x7FFFF74F5030 => 140737342558256
>
>
> As it is seen, there is 28 bytes of difference between them. 
> What is this behaviour exactly?

By the way,

writeln("Base Normal: ", &this);
writeln("Base Cast  : ", cast(size_t)&this);

Result:
Base Normal: 7FFFF74F5010
Base Cast  : 140737342558224

0x7FFFF74F5010 => 140737342558224

These are same. So, the issue is about addition process.