Windows 7 network bridge clones MAC address, what is it?

I have here an experimental Windows7 system which intercoects two etheet segments with network bridging.

It works like charm, but... it substitutes the MAC addresses of the forwarded packets with its own. This is not a bridge function, looks much more like an arp proxying router.

More exactly:

segment1 <--> Win7 <--> segment2

If I send a packet from segment1 to segment2, it is perfectly forwarded, but the receiver on segment2 sees the MAC address of the Win7, and not the MAC of the original sender!

Why is it? Is there any workaround? Maybe an alteative software?