I decided to model this in Excel, to see if it's a math thing or a perception thing (or a cheating thing).
Using the
Standard Array (15, 14, 13, 12, 10, 8), you end up with a total of 72.
For the
Point-Buy Method, I pulled up a complete list of all 65 valid ability scores that you can get using the Point Buy method (
Source). I put them into a spreadsheet, and summed each result (15+15+15+8+8+8=69, 13+13+13+12+12+12=75, and everything in between.) Then I added them together and averaged the result.
The average for Point Buy is 72.3, but that's moot: the player chooses a result between 69 and 75.
And for the 4d6 Method, I had Excel generate six stats using the 4d6 method, and add up the total of all six stats.
And then I had it repeat that calculation a total of 2,560 times, and then average the result.
The average result from the 4d6 method was 73.5.
So it does look like the 4d6 method will---on average, across thousands of iterations--generate stats that are about 1.5 points better than the Standard Array, and about 1 point higher than the
average value of Point Buy...but you can get results as high as 75 using Point-Buy, through careful selection of stats. (13, 13, 13, 13, 12, 11 gives you 75 points, which is much higher than any other method and it's 100% reliable.)
TL;DR: I think Point Buy is fairly well-balanced with the other methods. The advantage for point-buy is that the player can choose how over- or under-powered they want their stats to be, by as much as +/- 3 whole points from the standard array, while with the 4d6 method you are probably going to end up at +1.5.