I like the statistical analysis of things like this but I'm not sure I can trust the figures given. To say Browne had a slightly worse than 50/50 chance of sticking the ball into an open net from 6 yards and almost bang on central seems off. Also it happens so quickly live but I thought Nelson was stretching for his header and was past the back post whereas Ruffels was again quite central and running on to it without much competition from a defender.
Out of interest do you know how the figures are calculated and what metric they use? You mention Mackie's chance being harder as it's a header, is there some sort of rating applied to all headers for example? Pretty obscure but Josh Sargent scored a header for Bremen recently that would be almost impossible to miss here
Assuming there can't be a rating of 1, what would constitute a 0.9 for example? Not having a pop at the stats but genuinely interested to know.
The primary issue, currently with xG is the data available for league 1 and below. Whilst its an indicator, its not as reliable as those in the top 5 leagues.
Each xG model will evaluate each chance based on many variables. These could be:
Distance from goal
Type of pass assisting the attempt
Body part shot was taken with
Was the ball from a through pass/cross/set piece
Did the player dribble past players and shoot
How many defenders were between the attempt and goal
Is the player using his stronger foot
Different models include different variables...some now including the height shots were attempted from.
The real strength in xG is that it uses historical information from similar attempts to aid the probability. As such, its pretty dangerous to use xG in isolation.
In the specific case of Browne's goal, watching back, he is between the 6 yard box and penalty spot. There is a defender in front of goal as he strikes it. There are 5 defender in the box plus the keeper. The ball is a low cross from a dribble.
I suspect because Browne's attempt was from a cut back that reduced the xG.
With xG we already analyse the match in a similar way. For example, with the Mackie chance, 'he should have scored'. The ball is from a deep cross. He is between 2 defenders. he is 18 yards out. Its is a headers. All of these things reduce the probability of a goal. xG simply assess how many times out of ten the shot goes in from where the attempt was taken. Wyscout have the chance as 0.15.
The Sargent goal had an xG of 0.89 according to
understat
This is all pretty interesting stuff and is starting to shape how teams play. For example, what if you could train players to get in to better positions to take shots instead of constatl;y shooting from 30 yards? Man City are always scoring tap ins...the closer and more central you are to goal, the more likely to score. You can use the above to Improve finishing, game styles (e.g, crossing is inefficient. Cut back are better).
EDIT: just found this - this will do a far better job of explaining than myself!