Why is it that actual hours worked for a client is not equal to billable hours?
Are you really asking about "billable hours"? Or are you really asking about "billed hours". Because most of the rest of your question has to do with what gets billed to the client.
Clearly, time spent on a client matter should be billABLE to a client. And for firms that have a billABLE minimum, you would then get credit for this time. AFAIK, most big to medium sized firms have billABLE minimums.
Now, how much of the billABLE time is billED to the client is another question. There are three obvious factors that influence this: the attorney's bill rate (directly related to salary); the price that the client will bear (reasonable cap) or has agreed to pay (flat fee); and the amount of time that others bill to the matter.
So generally speaking, as long as you record time spent on client matters, you can meet your billABLE.
That said, IMHO there is one prosecution-specific factor that makes it hard to meet billABLE. And that's the umber of matters that we work on. Many of my days are spent doing very small tasks, lots of 0.1s and some 0.2s: calls from Examiners; quick client emails; lots of coordinating with my assistant. All of these tasks are billABLE. Yet many folks find it difficult to track small amounts of time spread across so many matters.
All of this dodges the separate question of whether a particular firm/partner has a culture of "don't record your billable time because I don't want to cut it".