There are several possibilities, including that the person reading the ECG knows the machine didn't detect anything objectionable, or enough of it to be of concern, or the machine was not hooked up properly, or was hooked up to you appropriately but not reading all leads correctly, or the machine's idiosyncrasies are well known and it shows what it reports while the tech knows better....and on and on it goes.
Ectopy is the generic name for premature ventricular and/or atrial complexes, PVCs and PACS for short. They are extra beats out of step with the otherwise normal rhythm, sinus rhythm, that the heart shows when not experiencing ectopy. A competent ECG reader would be able to tell which of those two it is in your case by the leads that show it. In your case, it is apparently atrial in origin. To put you immediately at ease, everyone, and I do mean everyone, has PACs now and then. They come and go. Most people have between 1-30 each day. It is when they number more than 77 each hour that their 'burden' on you is deemed to be excessive and an intervention may be discussed with an electrophysiologist....BUT....this depends on how symptomatic you are. If you are not bothered unduly by them, if you don't sense them well, if they don't rob you of sleep, if they don't make you anxious, and if they don't seem to be progressing in terms of lengths of run or frequency, then an electrophysiologist would probably want to do nothing. They are not lethal.