Why does the amount of sats in a lightning payment channel amounts vary if no txns are occuring?
Without detailed log files I can only guess the exact reason why you see this behavior. Two likely things come to my mind:

1.) Your node might have routed smaller payments. So in case you have outgoing capacity on one channel and incoming on another one someone might have used that to rout a payment. which would move some satoshi from one channel to the other

2.) If you opened the channels you are responsible for paying the fees in the case of a force close. These fees are taken away from the amount of satoshis which you own in your channel. Everytime you make a channel update (for example with a routing attampt - even if routing was not successfull) the fee estimator of your node and your peer try to estimate the onchain fees that your node would have to pay in case of a close. 

Since your node seems to be fairly new and since the onchain fees have been fluctuating a lot recently I am almost certain that my second guess is the reason for the behavior that you observed.