When you are running a split test between multiple variants (we'll just say two in this case; A and B) you will need to assign a weight to each. This weight is the percentage of your overall traffic that you'd like to see either A or B.
Now, every time a new visitor comes to your page, we'll randomly serve up a variant based on the weighting you have specified. If you've assigned both A and B a 50% weight, then this will be no different than a coin toss for each page request.
Something to note is that some imbalances may occur while the traffic to a page is still low. So, as in flipping a coin, you may get heads 5 times in a row.
We should also note that when a visitor first accesses your landing page, we deposit a cookie on that visitor's machine to track the view and to ensure that they see the same variant each time they return. If you visit your own page's URL, you might notice that you continually see the same variant no matter how many times you return or refresh your browser--this is because of the tracking cookie. To workaround this, just clear your browser's cache and try again but keep in mind that you will then be counted as another 'new' unique visitor.