Assume a site has two accounts "user122" and "user123". When a user logs in with "user122" and a password and it does not work: What happened? Did the user mistype the password or the username?
Assume a user got their username or password wrong. What is more likely, they got their username wrong in a way that happens to be the same as another user, or they got their password wrong?
Most people store passwords in their browser, which already makes the answer not that obvious.
For a rarely used web site, I honestly would have no idea if I registered as bonzini, pbonzini or bonzinip. Now my surname isn't particularly common, but smithj and jsmith might be easily confused.
I have a commonish firstlast@gmail account and frequently receive what is the obvious result of people in various parts of the country incorrectly providing my email to apply for jobs, order pizzas, and use other services.
It is absolutely common for users to supply the incorrect username/email.