There’s really no investigative difference between a TS and an SCI, it’s more in the adjudication and the paperwork to be signed. An SCI doesn’t mean much either until you’re actually “read on” to whatever requires it. They must have determined there may be a chance you might need it at some point and should be adjudicated accordingly now.
The Tier 5 investigation is the basic investigation for the high risk critical, TS, TS/SCI positions.
About 17 years ago, the BI process started combining requirements for similar investigation types - most had minor differences. This was the start of reducing the investigation types from more than 10 to the present 5, and hopefully one day to only three investigation types.
Anything is possible. But in cases it’s good news. You may have a certain qualification set that snagged someone’s attention. They have a plan for you. So they buttonholes you for that SCI project.