One of these users doesn't own the email address they have input. If it's the first one, I'm not sure how they could trigger a password reset without access, if its the second one, then I'm not sure why it's surprising you can have your account reset when you signup with someone elses email.
Assuming one John Smith creates an account as [email protected] ; and then another John Smith happens to create an account as [email protected] - and doesn't confirm the email right away (either the site doesn't ask for it; or doesn't really block any functionality - just keeps showing an annoying warning message). First John Smith can now easily take over the second account by triggering a change password flow - for shit and giggles.