DHCP Server Win 2016 full arp cache

So one of my peers setup a 2016 on a VLAN/Subnet. He came to me the network guy to try and figure out why only 40 or leases had been assigned to his VMs. The subnet is a /23 with 512 potential addresses and 20 or so reserved. There's no helper address here - just a party on the VLAN. So I hopped onto the DHCP server to make sure he configured the
IP address OK etc. But what surprised me was when I ran arp -a I found like 450 addresses associated with MACs already. The DHCP manager showed only 40 or so assigned. Anyone have a thought how all of these addresses might have gotten assigned to the mac addresses while the DHCP server failed to make a note of it? I pinged a few of the addresses from the arp -a and could not reach them. These are VDI desktops to-be. If the addresses are not pingable DHCP should go ahead and assign an address right? It's not concerned with what's in the arp cache right? Or no??