Originally Posted by
chaadster
You don't clearly identify the type of integrated headset on the Giant, but an integrated headset could have very little stack, but even "zero stack" internals have some height. Lots of units, like FSA Orbits, have 8.7mm top caps, so let's call it 10mm; maybe you've got a small spacer on there. So the Giant, then, with a 150mm head tube, could have had an effective stack of 160mm as well.
I started typing something along these lines but never finished the reply. I would add that some integrated headsets, like the Ritchey Carbon unit I bought for my CX1, has a top cap that's around 25mm tall, so tall that I was unable to use it with the same fork that was installed on my Gunnar Crosshairs with its non-integrated headset and ~25mm of spacers.
Lots of variables, even more if you aren't buying a factory bike and have your choice of headsets and the accompanying steerer tube length.