Why NETMASK of the alias interface is different then that of the physical interface if not specified while configuring ?

Issue

• Physical interface is getting IP- 172.24.76.X from dhcp and the netmask is of /25 or 255.255.255.128
• If an alias interface- like eth0:1 is configured on top of this physical interface of the same range as that of eth0 and NETMASK is not set then in ip a command output the NETMASK for this alias interface is seen as /16 or 255.255.0.0.