Bristol Central is a future seat created for the next election.
This potential change is because of the boundaries changing, there are lots of affluent and even very affluent people living in wards within those new boundaries. Places like Clifton, Redland and Cotham. People with liberal and even very liberal views but also people that don't like higher taxation and sharing "their money" with those less well off.
The Greens have essentially taken over the role of the lib Dems as a viable protest vote.
However the labour voters lost from the old boundaries (and transferred to Bristol North West) will likely secure Darren Jones and his successors a higher majority.