Why is mangling different for separate compilation?

[Jacob Carlborg via Digitalmars-d]

On 2016-02-27 04:51, Walter Bright wrote:

> It uses a sequence number to generate different ids for the unit tests.
> In the former, it's the 531st unit test, the latter, the first.

It sounds like the compiler is seeing different unit tests during the 
different compilation. But why is that the case.

-- 
/Jacob Carlborg