I'm guessing a bit here, but I'd say that it is probably in "width" & "weight" that you may find the biggest difference between advertised specs & your own measurements. Measuring folded size without pedals and weight without included extras such as fenders and rack could be a rather common way of making the folder sound more attractive.
Nearly all manufacturers of bikes are 'over-optimistic' - I am not sure Dahon are any worse. Some of the tricks used are.....
Weights without pedals (almost universal !!).
Sizes sometimes with bits removed eg pedals, saddle, and even seat post.
Where there is a range of frame sizes (and weights) the quoted sizes are all based on the smallest size.
Weights are measured without any accessories (eg mudguards, lights etc.)
Measurements are often rounded (up or down, depending which is best) Inches are good for this !
I guess we could all post our own bikes sizes (but then would we be optimistic) ;-) ?
FWIW...for my folders...
brompton quotes....22.2 x 21.5 x 9.8" and says "The size given is that of a bike with a saddle adaptor pin and with the left hand pedal removed; the folding pedal projects 20mm from this envelope. Depending on rider preference, the saddle may also lie outside this envelope." And Strida Quote..45" x 20" x 20" and 45" x 20" x 9" with folding handlebars
My brompton L3 is 23.5 x 23 x 11.5" 25lbs. Strida3 is 44.5 x 9 x 21" 20lbs (on old fishing scales). To be fair these sizes are not very important ... The brompton has an extended seat post and is better in small spaces, like a medium suitcase. The strida is long and thin with a few bits sticking out, they are equally portable. and although longer, the strida is better on public tansport - like a fat umbrella !!
Size is a difficult parameter to characterise on a folding bike since it is not a rectangular shape. You might ask your question on the forum at the dahon.com website. Dahon technical support seems to be pretty responsive there.