In theory no, they should both have access to the same content. Are there any examples of posts you see on PieFed but not Lemmy?

There are a bunch of reasons that come to mind. Sorting algorithms are different, for one, so you may see different posts even if the same ones are on both platforms. Secondly, PieFed has some powerful set up functions that allow you to pull communities from other instances or from the lemmyverse.net/communities list, rather than needing someone to find and subscribe. PieFed also recently scaled up it's mastodon integration, so now there's a "Microblogs" community where mastodon posts go if they weren't posted to a lemmy community, which may be content not available from Lemmy.