The firware actually has logic to 'expand' the short code into a repeatable sequence.

That is discussed in great technical detail in a document by Eigeny Eulanov in the Classic Pronto Files section right alonsigle Barry's document describing the Pronto Hex IR format.

I know there were issues with the 3000 firmware and these codes and we also had some issues with the 7000. Philips introduced the 'Duration' feature to help overcome some of these issues. Latest firmware and latest PENG will help on issues in the future.