Why is that? Well, since you are playing with a machine. Some software designed to randomly generate numbers and cards. Card counting online is way less of an exact science that card counting at casino. So due to the fact that the circumstances are various your strategy should change accordingly. You can use basic blackjack strategy, sometimes known as perfect blackjack, with your next move based on the cards you have and exactly what the dealership is showing. You hit, stand, split, draw or double-down according to a set of moves based entirely on what you have and what the dealer has. Such an approach has been mathematically tested and can, it is said, reduce the house edge to as low as one percent.
Strategy cards are available which set this out precisely. You could print one out and have it with you in all times whilst playing your blackjack online- something a high street casino will not allow you do- and follow the exactly coupled with a staking approach. You may decide on a fixed level stake per hand knowing that your basic strategy approach is reducing the house odds as much as possible or you may decide to opt for something else.