Gui settings change spot on screen with different resolutions

I have been messing around with changing how and item look in the hotbar. The item i was using was an apple but it was textured as a crosshair from a video game. The item was in the middle hotbar spot. So basically what i came to notice is that when you change the resolution of the screen the item will change position on the screen. I also tried changing the resolution of my laptop screen instead of just the minecraft tab but it was the same results. I originally made the settings on the resolution of 1920x1080 and it works just as planned on that resolution but when i change screen sizes the item will move to a spot i dont want it too