Interesting question. It appears it is currently sorted by merit left, then by user's merit score. You can test this yourself by looking at
this post. Among those who left +1 merit, those shown last have the lowest merit scores (around 10), and those shown first are those with the highest score (EFS is first among all the +1's because he has 1138 merit at the moment). I just left a +1 to see where I ended up, and I showed up between two users with 602 and 508 merit respectively (which makes sense since my score is 516).
It seems that the order is based on
1) number of Merit left, and then,
2) Activity, not user's Merit score.
Taking the post in the OP as an example, the first five users shown there are as follows:
User Activity Merit
Vod 2282 1244
Cyrus 1862 1000
mprep 1778 1109
minifrij 1582 1042
rickbig41 826 1011
We see that Cyrus' Merit score is lower than mprep, minifrij, and rickbig41, but Cyrus is displayed before them, since the Activity is higher than them.
I also thought a possibility that the ordering is based on the Merit score when they gave Merits and Cyrus' Merit score was higher than them, but this is not the case, I checked their Merit score was already higher than 1000 when they gave Merits.
Another example from Vod's post:
https://bitcointalk.org/index.php?topic=2818350.msg28856765#msg28856765The ordering of people who gave 5 Merits is
User Activity Merit
BitRentX 1082 1046
Astargath 630 503
Woshib 616 564
so Astargath's Merit score is lower than Woshib, but Activity makes sense with this ordering.
In addition to Activity and Merit, I checked their Rank, Posts, Position, Date Registered, but the Activity seems the one which can explain the current ordering.