I've been in the gambling industry (working) for 25 years, and prior to that grew up with a dad who sold gambling machines for a living, so I have plenty of experience. Whilst of course there are variants in the short term, over a period of time, the house will retain it's edge in various obvious, and not so obvious ways.
It's a fascinating subject and one that is a complex mix of mathematics, human emotions, Phycology and much more. Without getting into the vast complexities of how gambling venues/games/sites etc go about exactly retaining an edge, the simplest way to look at it is that the longer you play, the more probability, edge and maths work in the houses favor.
If you toss a coin five times, you may get 5 heads or 5 tails, toss it 100 and you will get more or less 50/50.