Actually, the only thing that one can deduce from the manufacturer's spec is that the maximum speed for a 109E that were delivered would be at least 475 kph. I would never venture so far as to say that the average of all delivered 109Es would be at 500 kph. We simply don't know. The 500 kph +/-25 kph is just a bandwidth resulting from negotiations between the client and the company.

So the average may be anywere between 475 kph and 525 kph when only consulting the specs.

What really matters is the real obtained speed value for each plane. To know where the average was one would have to measure the max speeds for every plane under the same conditions. As all planes were test flown there might have been abundant data albeit probably not under same conditions (weather, pilot, ...). They might also just satisfied themselves with proving that they can reach the 475 kph.

Now we only have a handfull of test data available, all with max speeds below 500 kph. So unless we find test data that shows that the 109 could reach 500+ kph I would assume that the average was somewhere between 475 and 494 kph.