These assume that everyone starts from the same point – namely, the ten-game points total that their predecessor left them. This means that everyone starts from a notional “100%”, whether the team had gained 0 points or 30 in the previous ten games. It’s a relative comparison of what difference the managers apparently made, in their first twelve games.
It’s worth noting that some of these took over mid-season, and some in the summer. I haven’t taken any account of that – my hypothesis would be that you would give *more* allowance to someone taking over mid-season.
The only reason there are two charts is to allow the scale of the managers who aren’t Terry Venables to expand a bit. Clicking on them will give you a bigger version.