They're not, you're probably looking at someone who taped it off telly and put it on a DVD-R. I have all 8 seasons, obtained via "other means", but the first four series were released to DVD that I know of (I have all four).
When UK Drama started up first they showed the entire series run. Up until then I had always thought it ended on series 4, and was delighted to realise it went further. Great stories too and an ending that.... well, I won't say in case you get to see it.
Sadly, when UKD repeated the series later they just went up to I think series 3 and then repeated, and when it moved to UK Gold the same thing, so I don't know about Yesterday, but would probably expect them to do the same, ie show only 2 or 3 series.
PM me if you want to get the other series...