IDA Pro displays certain buffer or padding above (at lower addresses) local variables in stack frame view. For instance:

Example 1. The following screen shot of stack frame view shows 12 bytes (included in the red box) buffer at

Exmaple 2. The following screen shot of a different stack frame view shows 12 bytes buffer again:

Why does IDA Pro display certain buffer or padding above (at lower addresses) local variables in stack frame view? Is it a coincidence that both views show exactly 12 bytes buffer? Is it something particular to certain calling convention?