Can someone please explain......

[Timo]

^That wouldn't preclude Worf from also being a Tactical Officer, though. Everybody from Data down floated during that season. But few people did so later on, so it might be worth the while to invent jobs for the first-season heroes that best match their seeming floating.

Timo Saloniemi

 

[shanejayell]

Could be a new development for the Luna Class ships, too.

 

[Christopher]

Could be a new development for the Luna Class ships, too.

No, because initially Keru was both security and tactical officer on Titan. Tuvok filled in at both jobs while Keru was injured, and once Keru recovered, he needed to be on reduced duty for a while, and Riker wanted Tuvok to stick around. So it was decided to split the security and tactical posts between them. It's not a class-wide practice, it was a personnel decision arising from the specific circumstances that occurred with this ship and these people.

While it's common for security and tactical positions to be filled by the same officer, it's not mandatory. It probably varies depending on the individual captain and officers involved.