What would be the reason for this?

Freqtrade is assuming the exchange's default fee for trades in dry-run (so for binance, thats 0.1%).
In reality, you will also be paying fees - given, these may be 0.075% if you're paying in BNB - however, considering slippage / sometimes unfilled orders, using a slightly higher fee in dry-run will make your dry-run more realistic.

Apart from that - Especially when aiming for very minimal profits per trade, you must be very careful how you price your trades (which side of the orderbook, which order type, ...) - as dry-run will always assume that an order fills within seconds - which in live may not the case if the order is priced poorly.

What would be the reason for this?

I think it would help with consistency between hyperopt, backtest and dry-run.

Freqtrade is assuming the exchange's default fee for trades in dry-run (so for binance, thats 0.1%).
In reality, you will also be paying fees - given, these may be 0.075% if you're paying in BNB - however, considering slippage / sometimes unfilled orders, using a slightly higher fee in dry-run will make your dry-run more realistic.

I agree, in the meantime I've set a 0.2% fee to account for that. Maybe this could be mentionned in the docs?

If you think this is not a useful improvement feel free to close the ticket :)

by setting fees to 0.2% - the effective fees will be 0.4% (0.2% for open, 0.2% for close).

I'm indecided on this - if someone feels like doing a PR for it i'm sure to look at it, but personally, i see no benefit - so myself i don't think i'll implement this any time soon.

@xmatthias I would like to take this issue up. I will look to set up the custom fee for dry run

sure, go for it 👍 (this is actually the main reason to label an issue as "good first issue")

I think it shouldn't be "too" hard - best look at the methods dry_run_order() and _store_dry_order() in exchange.py.

The setting (fee) should already be there for backtesting, so there's not "too much" additional setup required.

If you need any help, best submit a PR with "what you got" at that point - so we can look at it together.

@xmatthias that sounds great, thanks!!