The preview in the editor is made in HTML, so it loads quickly in the browser. The final document is rendered by the InDesign server. Because these are two different techniques, the preview may not match the final document 100%.
What can be different?
Text that doesn't fit: the preview cuts off text that runs outside the text frame.
Colors: the preview uses web colors, so colors can look slightly different from the PDF.
Text styles and symbols: the preview always shows bold, italic, underline and special characters, even when the font doesn't support them. In the final document, these may look different.
Effects: gradients, transparency and shadow effects made in InDesign aren't supported by the InDesign server.
How do you keep the difference small?
Make text frames large enough for the longest possible text, and set the Inset Spacing in InDesign.
Use CMYK colors (RGB also works) and add all colors to the Swatches. Don't use Pantone colors.
Make gradients and transparent shapes in Photoshop or Illustrator, and place them as a file.
Use Auto-fit for texts that can vary in length. Read How does auto-fit text work?
For all design rules, read What are the InDesign package do's and don'ts for the Template Studio?
Tip: Always check the final document of a new template before you make it available to users, especially when you use bold or italic text and special characters.
Want to see more detail in the preview? Read How do I enlarge the preview of a document made in the Template Studio?