The reason can be either a change in your emotional/physical state or measurement accuracy.
It does not mean that something is wrong with the app or the measurement is inaccurate. HRV is highly sensitive to even the slightest changes, including changes happening inside your body and external factors.
A lot of things happen to your body all the time: it digests food and adapts to external conditions. Your mental state and hormonal response are changing all the time too. That is why differences between back-to-back measurements are perfectly normal.