Just because something is a penny less and the people know it’s £18 - 1, Subconsciously, They take it in as £17 and some change. Even if they know that’s not the case. It’s how the mind work. Companies have been around a LOT longer then you and I; So you’ve got to wonder why the they still do it?.. Because it works 
Also, A lot of e-stores on the Internet are trying to feel like a brick and mortar shop but on the web. Emulating their pricing structure subconsciously makes you value the virtual item on the same scale as a real-life item.
Trust me, Companies don’t do this just to be an inconvenience. There are a whole multitude of reasons. And is it a bad thing?
You get your items for a penny less and their products have “Value” behind them, Where you value the digital item as if it were something you could physically touch and hold. :>