This can be explained by the spread, which if you don’t know, is the difference between the lowest ask (sell) price and highest bid (buy) price.
I couldn’t find it on the mobile app, but on the web app, if you open the graph of any instrument and click the little i button in the top right below your email address you can see the current sell and buy prices live. Depending on the instrument you can see spreads vary between 5c and 25c.
Hope this makes sense.