The way that email responses are quoted is dependent on the email program itself. So, in Mac Mail, for example, quoted originals are indented with a vertical bar along the side. The same might be shown in other programs with ">" symbols along each line. In Outlook, it could be any of a variety of ways.
In Mac Mail, there's a preference setting for "increase the quote level" for emails being quoted, so that each email in a chain of replies is indented further. But if I look at the raw source for such an email, it shows not the vertical bar but rather a ">" symbol next to each line. So Mail is interpreting the > symbol as an instruction to indent, despite the fact that the "real" email itself has > symbols.
Find the setting in Outlook and you may find the cause of what you're seeing. The problem is that the standard for quoting is the > symbol, so plain text versions will generally show the symbol, while interpreted formats will show whatever the email program is set to.