Why was the transaction fee more for a 20 mBTC transaction than for a 25 mBTC transaction? I just created a transaction to send 20 mBTC. The transaction fee was more than for a previous 25 mBTC transaction that I sent. Why is that?

I'm using the Bitcoin Wallet App, vers. 5.24 by the Bitcoin Wallet developers.
Transaction fees in Bitcoin are not dependent on the amount of value transferred, but on the amount of blockspace purchased.

When you try to send a transaction, your wallet will select a subset of the available unspent transaction outputs in your wallet to fund the transaction. In this particular case, it could have selected a larger number of inputs to fund the 20 mBTC transaction, and a smaller number of inputs to fund the 25 mBTC transaction. It could also be that the transaction fees just changed significantly between the two transactions which happens almost regularly over the course of the day or week.