The offset (ET) of a wheel is different dependent on the width of the wheel. Therefore if the Abarth wheels are 7" wide, as opposed to 6.5" wide on the Panda wheels the ET will be different as there is an extra 1/2 of wheel to accomdate.
ET is the distance from the wheels centre line to the back of the mounting face.
Though if you get it to be pretty close, ie 2-3mm it should all work out OK, but if there's a big difference you want it to be so that the wheel sits further inside the arch, ie ET50 as you can then use spacers to move it back out, there's nothing you can do if it sticks out to much ie ET10 as you can't remove the mounting surface as you would weaken the wheel.
All the info is normally stamped on the wheel 17x7J ET35 for example, J is not width, but J is type of bead seal for the rim and tyre, most standard wheels are J fitting apart from some speacil high performance tyres like the Nissan GTR which have a different fiting to ensure the tyre remains on the rim under high G force loading.