First of all, it does not show the current weather situation, but it gives a short time forecast (3-12 hours).
Then it needs several successive measures to identify pressure trending and so weather forecast.
An instant measure gives not enough information to do a good forecast.

Inaccuracy in the pressure measure may be due to other conditions:
- if you are in a tall building
- if you are changing considerably your elevation inside a building.

And, come on, it's only a barometer. No infallible forecast may be done with only a barometer!