Design mode compared with run mode
The designer shows an approximation. The browser shows the truth. Understanding the gap saves a lot of confusion.
What the designer cannot know
Styles loaded from a file, the size of the browser window, fonts installed on the visitor machine and data that only exists at runtime all change the final page. The designer guesses at these, and sometimes guesses wrong.
Trust the browser
When a layout matters, run the project and look at it in a real browser at the size your visitors use. Resize the window and watch what happens.
Rule of thumb. Use design mode to place controls. Use the browser to judge the result. Never sign off a layout from the designer alone.
Learn to move quickly between the two by running the project, and learn to debug when the browser result surprises you.