Yes they do. The hex is also affected by the level of optimization used.
Stay with C till you are good at it. Then you can learn ASM if you wish. Changing languages at this point would not be helpful. Note that had tom been using ASM I would have suggested he stay with it.
Given that the display can display all characters but you suspect that it has problems I would try this. Try writing each char on the display twice. For example to write 'hi', write the h, back up, write the h again, write the i, back up, write the i again. Not rocket science or a perfect solution but it may salvage the display with a simple software change.
You can try backspace to do the backup or use XY positioning.