6 channel AAC-LC audio bitrate is being capped to 1440 Kbps?

[BlohoJo]

I used fdkaac.exe 1.0.5 to encode a 6 channel RIFF64 wav file (5.1 audio @ 48,000 Hz) to 6 channel m4a AAC-LC using the following command line:

fdkaac.exe -b1536 -w20000 "file.wav" (256 Kbps per channel)

The resulting m4a file ends up being encoded at 1440 Kbps (240 Kbps per channel) instead of 1536.

According to page 14 of the documentation (aacEncoder.pdf), 240 Kbps is the minimum recommend bitrate for 6 channel AAC-LC, and it can support up to 800 Kbps:

(Note the laste two lines in the above image and below table clean-up:)

-------------------+------------------+-----------------------+------------+-------
AAC LC             | 8000 - 15999     | 11.025, 12.00, 16.00  | 12.00      | 1
AAC LC             | 16000 - 23999    |                16.00  | 16.00      | 1
AAC LC             | 24000 - 31999    |  16.00, 22.05, 24.00  | 24.00      | 1
AAC LC             | 32000 - 55999    |                32.00  | 32.00      | 1
AAC LC             | 56000 - 160000   |  32.00, 44.10, 48.00  | 44.10      | 1
AAC LC             | 160001 - 288000  |                48.00  | 48.00      | 1
-------------------+------------------+-----------------------+------------+-------
AAC LC             | 16000 - 23999    | 11.025, 12.00, 16.00  | 12.00      | 2
AAC LC             | 24000 - 31999    |                16.00  | 16.00      | 2
AAC LC             | 32000 - 39999    |  16.00, 22.05, 24.00  | 22.05      | 2
AAC LC             | 40000 - 95999    |                32.00  | 32.00      | 2
AAC LC             | 96000 - 111999   |  32.00, 44.10, 48.00  | 32.00      | 2
AAC LC             | 112000 - 320001  |  32.00, 44.10, 48.00  | 44.10      | 2
AAC LC             | 320002 - 576000  |                48.00  | 48.00      | 2
-------------------+------------------+-----------------------+------------+-------
AAC LC             | 160000 - 239999  |                32.00  | 32.00      | 5, 5.1
AAC LC             | 240000 - 279999  |  32.00, 44.10, 48.00  | 32.00      | 5, 5.1
AAC LC             | 280000 - 800000  |  32.00, 44.10, 48.00  | 44.10      | 5, 5.1
-----------------------------------------------------------------------------------

So why is it limiting it to 1440?

(I do know that 240 vs 256 Kbps will not be noticeable, and that's fine, but I still want to know why this is happening.)

I do apologize if I'm missing something simple and in that case I would hugely appreciate any information as to what I'm doing wrong!