I'm doing my shower wall now. I Went and picked up the thicker of the two hardibackers. I seem to remember from a tiling job years back that it was the same thickness as standard sheetrock. This time however, it seems it is listed as .42" which on a tape looks very much like 3/8". Sheetrock is 1/2" though. What am I missing?

 

You are not missing anything, you are just assuming incorrectly. Sheeted material is often not an even 1/2" or 3/4". 1/2" sheetrock is sold as 1/2" and it is actually 1/2". What most people call 1/2" plywood is actually 15/32" and what people call 3/4" is actually 23/32". Next time in the lumber section you'll see that the plywood is NOT labeled as being 1/2" or 3/4". 1/4" Hardie Backer is spec'd as being 1/4" but the thicker version is .42", it is your wrong assumption that it should be 1/2". Yes, it's very odd & annoying why some materials are nice round dimensions while others are odd fractions but as a builder it's something you have to be aware of and plan accordingly. If looking for a tile rated backer material and you want 1/2" thick you should get cement board. It is actually 1/2" and sold as 1/2"

 

I am curious why the .42"? It is not an even metric size either.