As per Elizabethan Gleanings, by F. W. Maitland in The English Historical Review - Vol. 15, No. 57 (Jan., 1900), pp. 120-124, the suffix "and so forth" (or "&c" respectively) has been first used by Queen Elizabeth I to circumnavigate the difficult question of religious affiliation (emphasis mine):
Has this phrase (sc. "and so forth") always been meaningless ? [...]
If we look at the book to which we naturally turn when we would study
the styles and titles of our English Kings, if we look at at Sir
Thomas Hardy's Introduction to the Charter Rolls, we shall observe
that the first sovereign who bears an "&c" is Queen Elizabeth. Now
let us for a moment place ourselves in the first days of her reign.
Shall we not be eager to know what this new queen will call herself, for
will not her style be a presage of her policy? No doubt she is by the
Grace of God of England, France and Ireland Queen. No doubt she is
Defender of the Faith, though we cannot be sure what faith she will
defend. But is that all ? Is she or is she not Supreme Head upon
earth of the Church of England and Ireland ?
The full difficulty of the question which this young lady had to face
as soon as she was safely queen may not be justly appreciated by our
"Styles" here refers to the "manner of address". Note that the above is speculation by the author of the essay, not necessarily fact.
The trivia given as the reason for the close votes is the claim in Wikipedia that
"European monarchs, who sometimes have lengthy titles due to dynastic
claims to territories accumulated over the centuries (and also as a
matter of prestige), often shorten their full titles by concluding it
with "et cetera"
The claim is not referenced (unless you count it as a reference that Yul Brynner in a movie based his usage of the phrase on a novel that features the king of Siam, which as a reference is absurd on a number of levels).