Under extreme climatic conditions a biennial plant may 'bolt' through the separate stages of its lifecycle in a very short period of time (eg.3 or 4 months instead of 2 years). This is quite common in vegetable or flower seedlings which were exposed to cold conditions before they were planted in the ground. This behaviour leads to many normally biennial plants being treated as annuals in some areas.
From a gardener's persepective, a plant's status as annual or perennial often varies based on location. For example, a perennial plant in a warm place might easily be grown as an annual plant in somewhere colder. This is because climatic conditions play a large role in determining the length of a plants life-cycle. If a normally biennial plant is grown in extremely harsh conditions it is likely to be treated as an annual because it will not survive the winter cold. Conversely, an annual grown under extremely favourable conditions may have such a highly successful propogation rate that it give the appearance of being bi- or perennial.