198.51.100.10 belongs to the TEST-NET-2 documentation block from RFC 5737. This example explicitly selects the RFC 3021 point-to-point interpretation. It does not claim that every /31 should be used in every network or that either documentation address is operational.
The mask leaves one address bit
The /31 mask is 255.255.255.254 and wildcard is 0.0.0.1. Clearing the final bit gives 198.51.100.10; setting it gives 198.51.100.11. There are 2^(32-31) = 2 addresses in the block. Those bit boundaries are the same regardless of deployment context.
RFC 3021 makes both values endpoints
In the explicitly selected point-to-point model, .10 and .11 identify the two ends, both are usable, and there is no directed broadcast address. Total and usable counts are both two. If the job were an ordinary LAN requiring two usable hosts, the calculator would choose /30 instead; it never infers point-to-point solely from the number two.