why have i got tongue and groove flooring up stairs but not down staires

I would imagine that originally all the boards in the house were non tongue and groove boards. And at some point the upstairs were replaced with newer tongue and groove boards. Theres prose and cons with both. the new T+G boards can be secret nailed through the tongue, and the board is able to expand and contract without opening up a gap in the summer months, so less drafts. however if you need to get to any utility pipes or cables below the floor you can't just lift 1 board (like you can with the old fashioned variety) to gain access to the joist void. You have to start taking the T+G boards up from the edge of a room, often also having to remove skirting boards.

Sounds alot like your upstairs has had the boards replaced over the years to newer T&G