re your first question: Nobody's stats are accurate, least of all W3Schools'.
The only people who really have accurate browser market share stats are likely to be Google, because everybody uses their services, and also their Analytics tracking code is on so many sites. But they don't publish their stats.
Everyone else is only going to be seeing a small portion of the global traffic, and their stats will be biased in favour of the browsers used by the people who visit the sites they monitor. Therefore, W3Schools' stats will be biased in favour of people who visit W3Schools.com. Since this is likely to be mainly web developers (albeit inexperienced ones), their stats will be biased in favour of the browsers that developers prefer. I don't know if they're getting stats from anywhere else as well, but wherever it's from, there will be a bias of one sort or another, and therefore the stats will be innaccurate.
The best source that I know of for global web stats is StatsCounter, which does seem to be fairly well respected as a source. But even then you should take the results with a pinch of salt.
Franlkly, the only stats that really matter are your own stats. What browsers do the visitors to your site use? That's the browser share you should be targetting.