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Hi J,

There are a few possible reasons for seeing the same MAC address for IP adresses from different subnets.

One possibility is Proxy ARP being use on a router or gateway device where that device gives its own MAC for remote IP addresses.

It is also possible to configure more than one IP address on a router interface.

On a Cisco router this looks like this:

interface GigagbitEthernet0/0
 ip address 198.51.100.1 255.255.255.0 secondary
 ip address 192.0.2.1 255.255.255.0
(...)

So you might see broadcast ARP traffic for adresses outside your own subnet in these cases.

There may be other reasons.

Hope this helps.

Cheers,