Readings are not live or constant. They are taken from the last time the modem finished its process. Check your Last Activity to tell when the battery and signal readings were taken. It is not uncommon to see batteries fluctuate slightly. If batteries fluctuate greatly it may indicate that one of the batteries has a dead cell. Cell signal will also fluctuate as it comes in waves.
Lithium batteries should always read 100%. The system measures the load they are pulling. If you see lithium batteries start to drain down it is time to change them, they are not pulling the full load.