Why is AC3 bitrate on DVDs limited to 448kbps?

I keep reading in DVD guides that the bitrate of AC3 audio on DVDs is limited to 448kbps, even though the maximum bitrate for 5.1 AC3 is 640kbps. Is this true? Is 640kbps considered out of spec, or just considered not proven to work reliably?

Bitrates of 512K and higher present problems to some multiplexers and players because the size of the audio frame exceeds the payload size of one DVD pack (2025 bytes). 512K frames are 2048 bytes, 576K frames are 2304 bytes, and 640K frames are 2560 bytes.

So how does DTS get away with it with 1536kbps streams?

A DTS audio frame has 1/3 the number of samples as AC3 (and 1/3 the duration), which would imply that 1536 take up the same space as a 512 AC3. But the actual bitrate of "1536 Kbps DTS" is 1509.75 Kbps, and that requires only 2012 bytes.