Pulled from Google:
"In general, the specifications for a USB 1.0 and 2.0 standard downstream port, delivers up to 500 mA or 0.5A. Also, with a USB 3.0, it can provide up to 900mA or 0.9A, which translates into 4.5 watts."
I would guess that the phone is being charged, however android auto is discharging the battery at a higher rate.
I would guess that the phone battery would last longer via the cable than if you were using a wireless connection.
Difference between your Octavia and Polo possibly down to the Octavia using USB 2.0 and the Polo using the USB 3.0 standard? Honestly dont know as I use a dedicated USB charger from the cigarette lighter.
I use this. I and my passengers all have android phones, charges them fine, no idea if its any good for Apple products.