Game of roulette itself is unbeatable using math. There Einstein was wright. No matter strategies or progressions, on the long run house wins. If it doesn't win from you personally, it has many other players that more then compensate for your humble ocasional win.
It happens because HE has origin on math itself. While there are 37 numbers on the wheel, payment for direct hit is only 35:1. This ratio does not change with any bet selection you may imply.
HE relay highly on the model of " perfect wheel" , where wheel itself behave as perfect random number generator. However, practice show that some wheels are not that "perfect", if not all of them, besides that things wear out with the time....
Result of this is very simple to understand, yet very dramatic in nature- some numbers hit more then basic probability would dictate, some hit less. There are physical reasons why numbers hit unevenly, be it normal wear of equipment, or result of sloppy maintenance ( most of the time)... in short, what it supposed to be topographically ( geometrically) perfect device, becomes not that even. It results in zones of numbers or individual numbers preforming differently then basic probability would make you to expect.
Differences are tiny, but comulative. To illustrate how these differences can affect your result, make a simple test. Collect 5-600 spins and simulate betting on 5 best numbers, do the same thing for 5 worst ones... compare results.
When choosing something other then numbers itself, outside bets for example, player has relatively same chances to choose mix between best and worst numbers, so results eventually level out and this there HE kick in. To prevent HE to have its affect on your bets, choose numbers wisely.