Playing with the underlying tool suggests that Cardiff and Swansea were both shedding jobs from 2004-2007 (Cardiff faster than Swansea). Swansea then too a big hit in 2008, but since 2009 Swansea has been booming (jobs-wise anyway) while Cardiff is still loosing jobs.

I'm not sure that actually explains anything, beyond suggesting that Swansea has turned around since the 2008 crash while Cardiff has carried on business as normal with a gradual decline, but it kept me occupied for 5 minutes.