This is the first I've heard of this. I'm pretty sure the P85 EPA sticker reflects an actual EPA test done by Tesla. If not, can you point to a source for this information or an EPA guideline that would exempt the P85/+ from EPA testing based upon number of units delivered? I just don't believe this, sorry.
I read the Code of Federal Regulations that documents what is required. I was looking to try and understand when Tesla would have to retest, at the time I believed they'd have to re-test every model year. What I found was far different than this. They only have to test a given vehicle configuration once. A vehicle configuration is made up of certain parts such as the engine, transmission, and so on. Any vehicle that shares the same components that make up the vehicle configuration is the same vehicle as far as the EPA is concerned. Those components that make up a vehicle configuration is not in the window sticker requirements (which have been updated for Electric Vehicles). Instead it points at the definitions used in the emission standards, which of course are not oriented toward an electric vehicle whatsoever. In fact the only pieces of a vehicle component that apply to a Model S is the engine (if you presume that is the motor) and the weight. As such from a vehicle configuration standpoint the S85, P85 and P85+ are all the same vehicle. They only differ by an inverter, suspension and wheels (none of which are part of the vehicle configuration as defined by the law). As far as the EPA is concerned these are merely options on the same vehicle. Tesla actually treats this the same way with respect to the resale value guarantee (though going one step further since the S85 is just an option of an S60). The S60 is separate here due to the weight differences and probably Tesla needing to explain the differences between an S60 and S85.
The question then becomes what option have to be on the test vehicle. Again the emission standards determine this. They list some options as being required on the test vehicle if they are offered all (e.g. Air conditioning). But most options are only required if they are expected to be equipped on more than 33% of the vehicles. As long as fewer than 33% of the vehicles have a given option they are not obligated to include it in the test vehicle. Since the P85 and P85+ are just options, as long as the percentage of those vehicles against the whole of S85, P85 and P85+ vehicles doesn't reach 33% then they aren't included on the test vehicle. However, if they would reach those numbers you shouldn't see a separate rating for them, rather you should see the S85 rating lower.
The P85D and the 85D however, each have a different motor configuration, thus (if assuming an engine is equivalent to the motor) each are to be treated as independent vehicle configurations. This explains why Tesla is quoting the test results for the 21" wheel configuration. It's probably safe to assume that more than 33% of the P85D vehicles shipped with 21" wheels. Thus the window sticker shows the results with the 21" wheels, even on cars with 19" wheels. Tesla however, initially published both on their website and has since updated it to show the 250 number with an asterisk saying 21" wheels decrease the range by 3%. My guess would be they got approval from the EPA to do this because it made the comparison on the website misleading since all the other vehicles had 19" wheels. But they still have to use the 21" wheel numbers on the window sticker.
I haven't bothered to cite the links to the CFR sections that define all this. Mostly because it's Christmas afternoon and I don't really want to spend Christmas digging through the CFR to find the proper cites. If you really want them I'll dig them up for you later, but I can assure you this is there. If you want to find it yourself, start with CFR Title 40, Part 600 and follow the references to the other parts of the regulations:
eCFR Code of Federal Regulations
Elon Musk
