They should be measured along the center line of the stem itself (parallel to the stem body), from the middle of the steerer tube clamp to the middle of the handlebar clamp (when installed). Middle in all respects, so from the middle of the steerer tube at a point half way down the clamping area of the stem.

The actual angle of the stem when installed is a product of both the stem angle and the steerer tube angle, so will vary between bikes. You would use that angle to calculate the actual horizontal reach. Perhaps with the 17 degree stems 3T assumed that they would be mounted on steerers with 73 degree head angles (which would give an effective angle of zero/90 degrees), so specified the lengths as the reaches given that assumption. I would have no problem with that as long as it was clearly stated in the specs.

<edit> On second thoughts, of course when installed on a 73 degree steerer and completely flat, the stem would actually have its "proper" length. I suppose you could take the horizontal measurement assuming that the steerer tube clamp was completely vertical and the stem sloping at 17 degrees (I think that's what you meant). It would make sense if that's what 3T did, although that's not the normal way of measuring. Again, it would be fine as long as they stated it clearly in the specs. Actually, maybe all of their stems were originally measured this way and now they have changed the lengths to make them fit the accepted standard? With the 6 degree stems that would make about 1mm difference on a 110mm stem..

I have seen some great Chinese fakes of 3T bars\stems is it def genuine ?

Both stems came from large and reputable U.K. online dealers (Chain Reaction Cycles and Wiggle), so I would be very surprised if they were fake.