Solar power AC DC power conversion
This is where physics is not commerce, and engineering comes somewhere in the middle.
The difference between a system rated at 6400 and 5760 is irrelevant, as far as you as an end user, or your appliances care. The important thing is the rated at bit, because neither of these systems will produce those numbers most, or indeed all, of the time, as insolation varies with season and across the day, the panels get dirty and old.
Maybe one system is rated at the DC cables off the roof and another is rated at the terminals of the inverter. The former is a bit naughty as you don't draw power for your appliances at the roof, you draw it at the inverter, which will produce less power due to inverter and cabling losses. You can assume however that the oik who installs the panels, or quotes you for their installation, doesn't know enough physics to explain what the ratings really mean, or whether they are being quoted at sensible or honest places in the system. Their standard playbook is to BS the customer until they stop asking questions.
One place where precise figures may matter is if you have an energy generation guarantee. You might purchase a first year, or 5 year, minimum kWh guarantee, and whether that pays out is going to be down to the last decimal place of the readings on your meter.