That is yet another GFS site.
I regularly use two GFS-based forecast sites and they often give noticeably dissimilar forecasts, due to the different algorithms used on the basic GFS data. Both sites have their strengths and weaknesses, apparent after a period of usage. The one I linked to above is run by meteorologists who also happen to be private pilots and is optimised for typical European conditions.
In the last week or two's unstable airmasses, the showers have developed pretty much where they said they would - I was able to plan and successfully complete a fairly big flight in the UK by taking the forecast at face value, altering routings by as little as 20nm to stay clear of predicted adverse weather. This sort of precision hasn't been available at low cost to the average GA pilot until quite recently.
Incidentally I wonder why they split the clouds into low medium and high? It is really a continuous spectrum, a lot of the time.
Maybe it's because that's the way the met. professionals have always classified cloud types? Base at: low 0-2Km, mid 2-7Km and high 5-12Km. Lets them use all the Latin names and prefixes they learnt on the course...
When I did the ATPL, those were the standard definitions (I think!)
I tend to think of low cloud being associated with the surface in some way, either by convection or orographic influence, medium cloud as not being convectively connected and high as completely crystalline in content. Then you get a Cb which spans all three categories.