Because you paid for the transfer fee in Ethereum and the transfer fee is classed as disposal so if the ETH has gone up in value you will have made a gain.
Simple example lets say fee was 0.0005 ETH and you originally bought it for $10, when you come to pay transfer fee of 0.0005 and its worth $30 the you will have made a capital gain of $20.
If you ran the Koinly full tax report on page 3 there will be a summary of expenses which includes Transfer fees for the year, these have not been offset against capital gains(screen shot attached).
I’m not sure if they are an allowable expense in your country but in the UK they are so I manually reduce my capital gains for the year by the amount of transfer fees.

I did submit a request for Koinly to add a feature to give us the option/setting to offset the transfer fees against the capital gains automatically but so far no response and they probably far too busy fixing bugs and implementing features that they consider more important,