Many differences are due to the fact that people often are able to understand otherwise mysterious utterances, because of the context and situation -- using intelligence rather than grammatical knowledge. For example, hearers may understand an utterance even if the utterance contains a word they hear for the first time (and hence they could never have produced such an utterance), provided the situation or context makes it clear what this word means. Thus `learning' often takes over from natural language understanding proper.
Alternatively, it may simply be the case that people understand sentences, they never utter, because they do not come up with the meaning in the first place. This situation might occur, either because they are not able to come up with the meaning (Einstein's case), or because they do not want to come up with that meaning (Dan Quayle's case) 1.2. The first time that Einstein explained to his colleagues the relativity theory they were probably able to understand him. However none of them would have been able to produce Einstein's utterances. As another example, consider the case where someone uses special stylistic effects. A hearer may recognize the social register associated with these effects; this thus will be part of the `meaning' of the utterances of that speaker. However, the hearer may belong to a different social class, and hence its language components will generally be instructed with a different `meaning' representation to that effect. Thus, it seems that some of the differences between understanding and production are not to be explained linguistically, but are due to a difference at another level of cognitive behavior.
Thus, maybe it is possible to maintain that the grammatical part of language understanding and production can be modeled by assuming it is based on a reversible grammar. On the other hand, if it is not possible to maintain this claim in its full generality, then I believe that the model proposed here provides a good starting point for a more realistic model of language behavior.