In short, every email client and browser renders HTML code differently from one another. Compare it to the numerous languages that are out there. While one joke may make sense and even be hilarious in one, it may directly translate into a grammatically incorrect statement that makes no sense at all in another.
The severity of your email anomalies will vary depending on the email client or browser and may or may not be rectified.
For example, there are known Gmail issues that can be easily fixed but Outlook issues that, unfortunately, cannot. Remember, though, these issues are limited to that particular email client. So just because you're seeing, for example, that some text is erroneously left justified in Outlook doesn't necessarily mean that it's going to appear that way in other email clients.
Fortunately, we have our Inbox Checker Tool to show you how your emails will appear in different email clients and browsers before sending it to your entire list. Click here to learn more.