The short answer is they are dealing with real numbers that are binary internally, and things must be done quickly.
So some numerical corners are probably being cut, that are normally not noticeable to the average web page viewer, that is if a page is designed so that it is not noticeable.
Go figure, I know, you would think that browsers would be more accurate, but I have found that even with the most careful and detailed control of my css, there seem to be rounding errors that play havoc with small details when displayed.
First, I would suggest you use em rather than px. I know you might have to start over with your css, but I have found it to be more reliable, screen to screen. Remember modern pages are not based on pixels. They are dynamic beasts that can be zoomed in and out, and squished narrower and still must output something reasonable.
Also I would suggest rethinking how you are laying out your page so such small details are not an issue. In carpentry the skilled artisan knows how to hide the rough edges with a molding.
I have a very accurate web page I use to generate to a special printed flyer. It was a royal pain to force html and css into something they didn't want to do, which was be accurate.
Also I can test my production site on the 5 major browsers, right on top of each other so I can see little differences when I switch between them, from one browser to another. They are all off just a little from each other in various ways, and really there is no way to make them accurate.
