The PHB p.180 specifies that during a round of combat, there are all manner of actions occurring during combat:
"In combat characters and monsters are in constant motion, often using movement and position to gain the upper hand."
It has been said that each round represents a theoretical six seconds in time during which a character finds an opportunity to take actions. The initiative roll is an abstraction representing the opportunity in the round that a participant gets into a position to execute an action (i.e. and take their "turn.") The idea that movement to open up opportunities is being taken throughout the round is also supported by the optional initiative rules in the DMG which allow for rolling initiative every round.
If movement is occurs throughout a round, this raises the question. When hidden opponents attack - when do they become visible? Do they become visible at the top of the round or only on their turn?
Using the top of the round would suggest that hidden opponents start moving and may not get an opportunity until later in the round before their action gets executed. Using only their turn would suggest they are able to stay hidden until they spring to make their attack.