David,

The reason that some ECU's have to have a read done is that there are different ECU model numbers in various cars (not all cars have the same ECU). Now this can be due to something simple as the manufacturer changing something in the car in mid production and therefore warrants a different ECU to accomodate the changes.
So, since there are various ECU's out there, and each ECU model number requires the file be written for it specifically there could be a possibility that we ahve not yet had a particular ECU in the shop and therefore do not have a file for it yet.
ECU's check themselves upon start-up and if the correct file is not in the car then the immobilizer will not allow the car to start.

The versions are all the same (no revisions yet) just written for the specific ECU.

Hope this answers your question.

Cheers,

Lanny