Ever since hypertension was first recognized as a risk factor for cardiovascular disease, it has been identified by measurements made in an office or clinic setting. However, it has been known for more than 50 years that in many individuals these readings tend to be higher than readings taken in other situations."

Source : Hypertension Research volume 30, pages 479–488.

This is the well-known 'White Coat Effect' - which is fairly easily explained due to the tension and anxiety that some patients feel when visiting doctor's surgeries etc..

Not so easily explained is the Masked Hypertension effect - in which a patient's blood pressure (BP) is consistently measured as lower in a medical environment.

The syndrome has yet to be explained. In addition, its prevalence is also unclear - some studies put the figure at between 10 - 20% of patients (for references, see paper linked above).

The implications of the concept that there are a substantial number of people in the general population who have undiagnosed and untreated hypertension which puts them at increased risk of cardiovascular disease require serious consideration."