Opera doesn't do fractions of a percentage which is why it fits in Opera. If you say 12.5% then in opera you get 12% but other browsers will round 12.5% to the nearest pixel.
Your measurements seem flawed as you have a container of 1000px width and then your content-container is 74% of that width
74% of 100px = 740px wide
Your feature boxes are 27.5% wide + 2px border + 40px padding + 1.1% margin
= 203.5 + 2 + 40 + 8.14 = 253.66 pixels
There are three of these in the content-container box and deducting the margin from the last one we arrive at a width of 752.78 pixels which is over 12 pixels too wide to fit.
Some browsers will round up and some will round down so to be safe you would need to make the elements add up to about 2 or 3 pixels smaller than the space allowed.
I don't understand why you are using percentage widths when your layout is 1000px wide to begin with. It would be easier to use pixel widths and know exactly where you stand down to the last pixel.
The only reason to use percentages in your case would be if you were going to change the main parent wrapper to a different pixel width and wanted the rest of the page to scale automatically.