Photoshop :: 16 Bit PSD Files Not Compressed?

Image compression in Photsohop CS6 doesn't seem to be effective on a 16 bit PSD when Maximize Compatibility is turned on.
 
For example, with Maximize Compatibility set to Always, if I create a new file 6144 x 4096 pixels x 16 bits/channel with a pure white background, and add a single text layer that says "This is a test", then save the file, it's 144 megabytes.  Zipping that same file into a .zip archive results in a massive reduction, to 245 kbytes.
 
Sure enough the saved file size goes down if I set Maximize Compatibility to Never. But even that can be hugely more compressed by zip (down to 88 kbytes).
 
This implies the failure to compress is happening on the flattened preview.
 
If I change to 8 bits/channel and save that same document either way it's about two and a half megabytes, differing only a couple hundred kbytes between Maximize Compatibility Always vs. Never.  In this case, apparently the flattened preview is being compressed nicely.
 
These are my file handling preferences.  Note that I do NOT have [ ] Disable Compression checked.
 
Doing a bit of spot testing, I believe this is nothing new - it has been the same in recent past versions (e.g., Ps CS5, CS4).  I've just never noticed that saving 16 bits/channel data yielded such huge files (because I always diaabled compression anyway), but apparently it has been the same all along.
 
No problem here - this is just a curiosity to me, as I have a fast disk array and these saves are all quick enough.  I choose the "Always" maximize compatibility setting because I have a viewer (IrfanView) that I use for quick previews that won't read files saved with Maximize Compatility disabled.
 
I suppose it does beg the question:  What prevents Photoshop from doing the Flate compression on the flattened preview in a 16 bits/channel PSD file?  Was there an old version that couldn't decompress such files?