The #expr stuff is basically a reinvention of the FedoraVersionNumber
template; that template transcludes CurrentFedoraVersion, so if we just
substitute FedoraVersionNumber (or FedoraVersion) it doesn't solve the
problem, as the underlying CurrentFedoraVersion usage is still a
*transclusion*, so the number would still change in result pages. Which
is what we're trying to avoid.
If you're interested in all the details, ask and I'll rewrite the long
answer. :)