There are often unsold old stock that get registered on later plates. There were quite a number of K reg Mk2 Golf's around, even tho' the Mk3 was launched on a J plate.
As with older models of Golf, the cabriolet often spanned two generations. The Mk1 cab was produced alongside the Mk2 hatch and the Mk3 cab continued alongside the Mk4 hatch (albeit with a Mk4 nose job)