I don't think I was clear.
The bug is that it isn't working properly.
This situation was caused by:
1) editing "Media Contract"
2) going back to the financial page
3) going back to the editing page, where "Media Revenue This Year" appeared to have automatically updated, but in fact had not
I am aware that those in the know could avoid this situation, but it is generally considered a bug if a user can break it through any normal action, and that is the case, I think.