Consider this scenario. Project A and B give the same amount of credits per day.
A user in project A finds a way to make the app twice as fast. The project uses
fixed credits, so he gets twice the credits per day with his modified app; which
is OK since he's doing twice the science. He sends his modifications to the
project admins, who decide to release the new app officially, to all users.
Option 1: credits per day stay the same (ie. credits per WU are reduced to
compensate) in order to keep cross-project parity, even though everybody is
doing twice the science. User is discouraged from contributing further
optimizations; he gets more credits by keeping his modifications for himself! A
user who crunched for two months with the old app would have the same credit
than one who crunches for two months after the optimized app is made available,
yet the latter did twice the work. Doesn't sound fair to me.
Option 2: whole project gives the same credits per WU as before, which means
twice the credits per day. Project credits per day don't match project B
anymore. Project B had already worked on app optimization before project even
went public, and it's as optimized as it can get, so A would forever have more
than B (there's still room for more speedups in A!).