The "Preferred Name" is the name by which a person is or was actually known, when it differs from the person's formal name. The "Proper First Name" sometimes has quirks of spelling (after all, the poor baby was inarticulate and its parents may have been illiterate) and is often not very revealing.
For example, many parish scribes habitually used the spelling Johannis instead of Johannes. There are over 1000 of them on WikiTree, mostly with Johannes as preferred name. But in lists of children, siblings etc., it still shows as Johannis.
One would think that when there a choice between versions of a name, the Preferred name would be preferred.