Buy and sell rate change

Please can someone direct me to where I can understand the mechanism that dictates the difference between the stock market buy price and the sell price?

I understand that there is normally going to be a difference and the sell is expected to be lower, that’s just business. I know it changes as well.

But I’ve noticed that there is a growing range of late between these, up to 10p on some shares. This makes a heck of a difference and often I’ve seen my average is over the buy price but the sell price pushing it under.

It became even more complex today when I was looking at my VOD - Vodafone stock. At one point the sell was actually more than the buy, only by a fraction but surprised me all the same.

I’ve leant a heck of a lot since I started trading six months ago, but this defeats my logic.



Thank you for that, useful.