At 10pm, a homeowner with water dripping through the kitchen ceiling calls a plumber. If it rings out, they call the next one, and the next, until somebody answers.
Whoever picks up gets the emergency call, and often the water heater or repipe conversation later.
Meanwhile, the daytime calls you miss because you're under a sink add up quietly. None of them show up in a report.
Emergency calls after hours
Burst pipes, backed-up drains and no hot water don't wait for office hours, and the caller won't either.
Calls while you're on a job
In a small shop, the person who answers the phone is often the person holding the wrench.
Water-heater and repipe estimates
A bigger estimate goes out once, then nobody follows up, and the homeowner books whoever did.
Web requests that wait
Drain-cleaning and service requests from your website sit until someone gets back to the office.