I think this is way easier explained with an example. 

Say you had an armor stand at Spot A, and you made a command block line that teleported the armor stand to Spot B, then detected if the block underneath it was stone. If you put stone under Spot B, it would come out as true. 

Now say you still had the armor stand at Spot A, but you had a command line that teleported the armor stand to Spot B, then detected if an pig was within 10 blocks. If you put a pig 10 blocks within Spot B, it would come out as false. If you put a pig within 10 blocks of Spot A, it would come out as true. 

If the command blocks teleported the armor stand in one tick, then detected for a pig in the next tick, it would work. It only breaks when the teleportation and the detection are right after another in the same tick. 

The same effect happens when trying to summon something at the armor stand (it would summon at the spot were it was before teleportation).