2012 measurements are here:
http://salsacycles.com/bikes/2012_horsethief
ETT comes out as you’d expect if you use reach/stack/SA to calculate it. Note that it changes a little based on fork length (3mm for 20mm of fork on their charts)
Thing is, 10mm out is easily within error if the bike wasn’t on a level surface or the measurement wasn’t dead level. So whilst that could account for your error, it could also account for mine measuring up the same and it’s not really!
Having said that, on paper I worked out the bars/saddle distance to be the same (with 60mm stem) as my XC bike, and that’s what it’s ended up as (and that’s an easier measurement to take).