That calculator isn't accounting for much but the direct input. It doesn't "understand" how the things are being added. Refer to Wolfram Alpha here (you aren't likely to find a better calculator anywhere on the planet). Through direct addition of percentage points, we get 70%. Through a relative percent increase (which is what this is), we get 60%. Through a compounded interest calculation (which this definitely is not), we get 80%. http://www.wolframalpha.com/input/?i=50%25%2B20%25
I don't think it's an issue of the calculator being wrong, and I'm almost certain that us being wrong is not the issue. I think you're just misunderstanding what exactly the game is doing, and I think you need to brush up on how percentages actually work and what they mean.
Unless I'm looking at the wrong Wiki page, it doesn't go into any detail about the calculations. In either case, I take the Wiki pages with a grain of salt. They're usually pretty accurate, but they're not official by any means.
The sneak multiplier is a total mess, that's absolutely true. That's quite a bit different though. Taken directly according to what the perks say and how they add or multiply the bonus, it's completely mathematically valid, when taken as a series of small gains, as opposed to a cumulative gain. I posted a thread about it a while back and actually worked out how the 6.3x sneak attack multiplier works. I worked out the other possible combinations (there are 9), but didn't take the time to type them all out. They're mathematically valid when taken as single discrete calculations that occur one at a time. They're not really mathematically valid when viewed as a single calculation though.
A single word can make all the difference. Philosophers have spent centuries ironing out the finer points of the difference that one word can make in if/then statements.
That's been one of my favorite little logic puzzles for a really long time. I remember in 6th grade my math teacher asked us that as an extra credit question on a test or a quiz. I was the only one who got the right answer. I've since gotten into actual debates with people who are just absolutely convinced that it's a paradox and that mathematics just utterly breaks down with it. I've tried breaking down the entire scenario for them and showing them, mathematically, that there is no missing dollar, but some people just seem to have minds that don't work that way.