HR monitors are pulse counters; in that sense, they can’t be calibrated wrong - but they sure can count the wrong pulses. For a strap monitor like yours, the typical issue is the strap being dry; it then tends to pick up other pulses than your heart, such as cadence for runners, and sometimes upper body movements associated with pedaling for cyclists. A bad battery or battery contacts can cause the device to operate erratically, causing HR spikes up/down.
First thing I’d do would be to look at cadence in the areas where recorded HR appears off. If cadence is equal to HR (running) or half of HR (cycling), you have your answer. Make sure the contacts are well wetted before starting; if needed, try one of the gels sold for that purpose. Some synthetic materials behave worse with some straps than other combos, so that’s another solution to look at.