One possible answer is that the speaker is more concerned with the pure certainty of the action happening than any volitional aspect that might be implied by the use of WILL by itself. This idea can be illustrated more clearly in the following examples. If someone says I'd like to know what Andrea thinks about this, responses might be:
1. I'll see her tomorrow; I'll ask her.
2. I'm seeing her tomorrow. I'll ask her.
3. I'm going to see her tomorrow. I'll ask her.
4. I’ll be seeing her tomorrow. I'll ask her.
In all four examples, the I'll ask her indicates the speaker's willingness (confirmed by the context) In the first half of the utterance,  indicates the speaker's willingness to see her,  the speaker's knowledge of an arrangement already made to see her,  the speaker's awareness of present evidence of the future meeting and  the speaker's simple presentation of the fact of the future meeting. It is claimed by some writers, with some justification, that the use of WILL BE ....-ING implies, by its lack of reference to intention, volition or arrangement, a 'casual' future.