Most times a receptionist is not a clinician , even if they were their diagnostic skills would have to be incredible to diagnose over the phone with no x Ray, no clinical exam etc.
If you book a car in for a repair the technician knows exactly how long it will take , barring things like seized nuts etc.
Unfortunately teeth vary immensely are attached to human beings who vary immensely . A routine filling can become not routine because
a an x Ray is only an inaccurate 2d representation of a 3D object . So decay can be much more extensive and a simple filling may turn into the start of root treatment or an extraction etc
b people are different . Someone might come in for a tooth out not having eaten before , so will need glucose and time to recover before treatment can start.
Someone might not numb up well enough first time, they may need longer to numb, or a different technique or a different anaesthetic
Someone might be very,very nervous and need a lot more time to become comfortable with treatment than was forecast.
c you might spot something that needs doing whilst you are working on another tooth that didn't come up in the x Ray or examination.
d you get people in (normally of the older generation) for a check up with eg a massive swelling , who didn't come in earlier because "I didn't want to bother you and I know you're busy" .
e people book in for a problem that seems minor "a chipped tooth" and it turns out they've broken a crown off that needs longer and more complicated treatment . It takes time to get equipment out and then everything cleaned away and sterilised afterwards before the next patient.
In other words there are many reasons why a dentist runs late and cannot always be on time. As a matter of course you should be informed so you can rebook if it's not convenient to wait but it is not a deliberate thing. Surveys show keeping to time is one of the main stressors in dentistry.
Most dental surgeries work with no breaks , other than lunch , during the working day and lunch is used to keep up with paperwork, referrals , letters etc . Many colleagues and nurses complain they don't even have time to wee. It's no one's choice to run late and the dental team will be incredibly stressed as well.