Coinbase One hidden costs?

I have the Coinbase One subscription and thereby saving transaction fees. I made two BTC buys at the exect same time:
-50 Euro using Coinbase One giving me 0.0008681 BTC
-50 Euro using Coinbase Advanced Instant Market order giving me 0.00087326 BTC

Coinbase One giving me 0,6% less BTC for the same money???

Coinbase Advanced do not benefit from the Coinbase One subscription, so I ended ud getting BTC for €49.80 out the €50 when the fee had been paid.

The difference was obvious already when preparing the orders, the price per bitcoin was higher at Coinbase One. I made the purchases exactly at the same time, so it is not only due to the price changing.

Can anyone explain, thanks?