Or made at a different time,even a few seconds means you'll get a little different reading 
Yeah maybe. I tried refreshing those two sites at various times and they're always different. But you could be right.
It is highly unlikely they recalculate the difficulty on every single page refresh. Both sites likely calculate and STORE the difficulty periodically (say once every hour) and page refreshes simply pull the stored value. Unless both sites use the exact same timestamps and have the exact same blocks in memory at the exact same time they are going to get different values. (Different Input -> Different Output).
