This is called dynamic pricing and is a widespread method for online shops to maximise their profits. It is often done when other shops offer an identical item at a lower price, and as soon as the other shop raises the price again, the competitor shop follows suit the next day at the latest. It also happens when you want to buy something on the wrong day, there are days when things are generally cheaper. That is a true science. The price chart at Digitec provides a little more transparency, I would usually pay attention to it, i.e. if something was cheaper once and is expensive today, don't buy it, it will definitely be cheaper again, just wait and see.