Photoshop: Why does LZW compression in TIFF sometimes increase file size?

Adobe PhotoShop 5.1 Version:
12.1 (12.1x20110328 [20110328.r.145 2011/03/28:10:30:00 cutoff; r branch]) x64
Operating System: Mac OS 10.7.2

When storing a TIFF image using the LZW compression, the file size consistently increases by some 20-30%! I'm using TIFF mode RRG 16 bit. I thought the resulting file-size was supposed to decrease when using this option?. Storing it once more w/ None compression will again reduce the file size to the smaller original state.

Any ideas why this happens?
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