The trick here, I think (or at least the way I look at it), is to realize that, as its name actually implies, the present perfect continuous (i.e. "I have been working") is actually a present tense. What this means is that when you use it, contrary to how most people think about it, you are talking about the present, you are not talking about the past.
That is, when you say "I have been working", you are not actually technically talking about working in the past. What you are doing is actually describing your present state, and saying that your present state is a state that happens as the result of previously working, that is:
I am currently in a state that is the result of having worked.
Now, the reason that this usually implies that something has been happening up until right now (or at least recently) is because one's state generally changes due to many factors over time, so the longer it has been since the cause of the state occurred, the less likely it is that something would still be considered to be in that state. There's some ambiguity in this, though, because it may depend a bit on what sorts of things one would consider to change what sorts of states. For example, you can say "I have been working" some time after you actually finished working (and went home, for example), but generally if you did some other significant activity (such as sleeping, or playing) since then, then you wouldn't consider your present state to still be "a state that's the result of having been working".
This then gets into the area of what's considered a "significant activity", which, even with the same verb, can depend a lot on context, of course. If you are actually at work, and talking with somebody about what you've been doing there, the relevant options are likely more limited (i.e. the implied question is probably something like "are you working or are you goofing off?"), and so it's implied if you're in the state of having been working (present perfect continuous) that that means you haven't been doing anything like "goofing off" between when you were working and now, and since there aren't a lot of other options, that means you must have been working up until right now.
Because we are talking about the present state, rather than the past, in some ways this is similar to "have become", which is also talking about the present (contrasted with something in the past). The big difference is that "have become" refers to a persistent change in ones nature (i.e. a much more fundamental and long-lasting change), whereas "have been" just refers to one's current state (which can change all the time for many reasons). Note that "have become" is the present perfect tense, whereas "have been verbing" is the present perfect continuous, so obviously, they are going to share some similarities due to their common present perfectness.
