Because users read items from left to right, the priority direction for reading items is stronger horizontally than vertically.

Items in a top navigation do not have equal weight. The leftmost items carry more visual weight than other items because of its placement in the primary optical area (top left). Items in the top left area get more exposure and are often seen as more important than other items.

you will have certain items with higher priority than others. Because the user’s topic of interest is more limited in this context, placing items in a top navigation allows users to find what they want faster and easier.