welcome to the forum. The minimum allowable car weight is 605Kg. When you start your race it will be much heavier. When you finish your race it will be much lighter probably around 608Kg - 611Kg depending on how much fuel you have left. This is quite normal. It sounds to me that you are verifying your settings from the start of a race or with a lot of fuel on board. I reckon each lap of fuel is worth approx 2Kgs - 3Kgs depending on the circuit.
If someone were to cheat and make the car much lighter it would go under 605Kg and their race file would be disqualified. Hope that answers your first question.
As for the bhp - everytime you start a new session GP3 gives you a random bhp value between 795bhp and 800bhp. To be honest I would be amazed if you could tell the difference between such small values and I have set PBs with 795bhp. Again it's to stop people from cheating so you can't raise this value manually with one of the editors out there.
Sometimes you get lucky and you may get 799bhp but it hardly makes any difference at all. Maybe it even creates more wheel spin? Who really knows?