Expectation bias? Same reason something sounds better when you change cables even though the result is objectively the same.
The Xiaomi power bank should tell you everything you need to know about this experience. Your mains power has one very crucial attribute: It's 50/60Hz AC. If now you plug your ordinary mobile phone charger in have one of two results: Either you hear a hum, or you don't. If you don't, then your performance *could* in edge cases be limited by the noise of the switchmode powersupply.
Along comes batteries. They are pure right? Well... no. That Xiaomi power bank isn't just some batteries, it's a power source followed by a switchmode regulator, built to the cheapest possible price and meeting exactly the same USB power standards as required from the spec.
IF this all made a difference then it would be expected that the phone charger and the power bank perform equally poorly.
