in a word - impedance matching. If you look at the Sirenza datasheets, the Z pieces are stripline (0.54mm on 0.032" board, of a certain type). Unfortunately (at shorter wavelengths in particular) this stripline acts as an inductor (mainly) and so becomes part of the circuit. Ladyada's design does not conform to the appropriate Z lengths, which as you can see are expressed in wavelengths.
This is why RF design is described as magic. Unfortunately there are also issues such as solder fillet thickness, and constructive/destructive rf reflections going on at multiple places, which can only be measured accurately with a $40,000 network analyser.
So in the end - trial and error with a spectrum analyser on your unit is the best (cheapest) way to improve signal strength - I found slightly different values than Ladyada, but maybe that's because I used different soldering technique, or maybe I was focussing on slightly different fqs.
hope this helps.