Isn't the the purpose of a YUP? The OP doesn't say what Fare classes he sees (perhaps because he doesn't know where to look). Most of the time a "restricted first" ticket really is a YUP, and as somebody else said, there's no advantage to full fare over a YUP if you know the dates won't change.

Of course, if you search for a flight with no A or P inventory, the system will fall back to full fare F, which could also be the issue.

Unfortunately, without some more info from the OP, we'll never know.