Not....But and parallelism

[at.ouchen]

Good day
Accordin to the rule "NOT A but B" is a parallelism structure in which A should be similar to B.
But in page 160 of Manhattan SC I've found out the following sentence.
The agency is NOT a fully independent entity BUT INSTEAD derives its authority from Congress. (note that the verbs is and derives are parallel)
According to the inside rule "a fully independant entity" should be parallel to other noun or noun phrase, not a clause (the cas here), and second how can is be parallel to derives in this sentence??!!!
Many thanks in advance.

[RonPurewal]

If you read that sentence as "thing #1, BUT thing #2", then it's fine.

Thing #1 just happens to contain "not".