I think they add little extra for joint care such as salmon oil maybe, I mentioned what I fed my dog to the vet as to whether to change him to senior (he is 11) and she said that as his weight was fine and mobility ok to leave him as he was - and her general attidude was he looked well on what he was on so why change. Senior foods seem to add glucosamine too and supplements for immume system etc..
The only thing I do is add Salmon Oil which I got from Wilko, I don't give as much as it says on the bottle, but he loves fishie food so it does go down(if I put a full dessert spoon on he leaves it)....his is a fussy eater at times and a hoover at others. He has a mix of dry and wet and occasionally I give him a senior wet food as well as the other chicken, salmon and fish variety of the wet food brand I use.
Personally I would do what you are doing and next time Pebbles is at the vet mention it to them.
Not sure whether I would class a 7- 8 year old as senior - I thought 9 to 10, but see what other say to that