It would appear in surfacing if the final surface produced also knits together within that feature (certain types of trim for example) and it was the only/final gap as such in the surfacing it results in a fully knitted model. This model has no gaps but is listed as a surface entity. As all the surfaces are knitted together there would appear to be no function in solidworks to convert this to a solid (or rather SW just to recognise it as a solid which it is).
I know the work around - make sure the last feature while surfacing doesn't knit/or off set a surface zero and delete the original and knit to solid. Which either is all a bit a of a mess around.
So am I missing something really simple here which makes SW know that it is now really a solid??