Balance in Sphinx

Why does your balance fluctuate?

The balance fluctuates because commit fees for channels get renegotiated as the cost to publish to the bitcoin blockchain changes. When the commit fees rise (BTC fees become more expensive) some of your local balance is reserved to meet those fees, resulting in your local balance dropping. When fees drop the reverse happens.

How does your Sphinx balance is calculated?

Channel Capacity - Remote Balance - Channel Reserve - Commitment Fee = Disposable Balance (Sphinx Balance)

Since the Commitment Fee is not a fixed variable in the calculation, your Sphinx Balance can increase or decrease.