Am I exposed to WebRTC leaks if my IP resulting from WebRTC is the IP of my VPN server with the last digit?

I am connecting to the website of ExpressVPN that checks for WebRTC IP leaks.

My IP as resulting from that website is the same IP of the VPN server I am connected to but with the last digit augmented by 1.

That is, if IP address of my VPN server is

A.B.C.D (e.g. 1.2.3.4)

the IP as detected from that website is

A.B.C.(D+1) (e.g. 1.2.3.5)

What does this result mean?

Why is the last digit augmented by one?

Am I exposed to WebRTC leaks?

EDIT: More information:
Actually A.B.C.D is the IP address I have on my openvpn configuration files.
That is, inside the openvpn configuration file there is written
remote A.B.C.D 443
But when I connect to say whatismyipaddress.com/ I see A.B.C.(D+1)