There isn't a simple answer to this question. Consistency in email deliverability can be a complicated issue to fully solve. However, to answer your question directly, it is because each email client renders emails slightly differently, thus creating variations in a real send versus a preview when designing.
The most common practice for consistency in emails is building everything in a table based layout. For people with web development knowledge, this is sort of like designing a webpage 10 years ago, but its the most tried and true methods for clean emails. There is a number of other things that don't work in emails as easily as they would when building other things, and if you are new to it it can be frustrating.
Here is a decent guide to the fundamentals of designing and developing emails I recommend. To troubleshoot your specific issues on this email, you'd need to post your code for it.
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