FormHub and ODK sites share a common XLSForm converter.
Other groups have added features for different client requirements and
either have not been required by their clients to contribute the code back
to the main tree, do not have the free time to do so on their own, or see
their extensions as value-added proprietary features.
The set of functions within calculated expressions and the set of
appearance attributes varies greatly across different implementations and
is constantly being expanded. The same issues apply here as to the XLSForm
standardization, but there is much greater variability in the extensions
different groups have added to their own code.