You may be less relaxed with your doctor than at home. This is what we call "the white coat effect". When you are nervous, blood pressure increases. A systolic pressure less than 15 mmHg is normal in a relaxing environment.
Blood pressure varies during the day. Variations of as much as 30 mmHg can happen over a short time due to resumption of physical activity between measurements (walking, going up or down stairs, bending over…) or disturbances in your environment (noise, stress, fatigue, agitation…). We recommend measuring your blood pressure in the same time slot and under similar conditions, for example in the morning before breakfast and/or the evening one hour after dinner. See the precautions of use to be respected.
Each manufacturer has its own principles for detection of blood pressure. A difference between the results provided by the blood pressure monitor used at home and the doctor's measurement can be explained by the difference of observation, in the same way as what happens when two doctors listen to the same patient with their stethoscopes and get different results.
*credit to Sylvain of iHealth Labs EU