Ava measures the temperature at your wrist so it will display a different temperature reading than you would get from an oral thermometer - even so, it is still a reflection of your BBT.
Your core body temperature reaches its lowest point during the night while your extremities heat up. During the day, the reverse is true: your core body temperature increases while your extremities cool down. Both your core body temperature and your extremities follow the same pattern of rising by one half a degree after ovulation.
But more importantly, temperature is only one of the several parameters that Ava measures. It also it tracks neurological, thermal, cardiovascular, and other parameters to gain a more complete picture of your fertility than temperature alone can provide.