I guess that they decided that if the city of St. Augustine was going to get a coat of arms, the county (in which the city is located) should, too.
Don Vicente de Cadenas y Vicent
Certification of the Coat of Arms
Which Corresponds to the Use of the
County of Saint Johns, Florida
Madrid, 12th of October of 1991
Vicente de Cadenas y Vicent, de Gaztanaga y Nogues, Herald, King of Arms, Dean of the Corps of Heralds
I CERTIFY: That on petition of the desires expressed by the authorities of Saint Johns County, in Florida (United States) which desires to perpetuate the historic memory of the population and founding of that County by the subjects of the Kings of Spain and in order that the memory should remain steadfast, the drawing up of a Blazon is solicited which for fundamental symbols of the same bring those events to mind adapting them to the Science of Blazon, and as a consequence and by virtue of the desires expressed by the already cited authorities that in its field they wish to reflect three aspects; two of them of historic character and one symbolic, taking for it (the Blazon) a castle as an allusion to the Garrison which was in the County, the Arms of Castile and Leon as a remembrance of the epoch of its discovery and union to the Crown of Spain, and a direct allusion to the name of the County referred to, and as a consequence, it stands organized and composed in the following manner;
In a field of green an Agnus Dei of silver, suspended on the dexter side of the Agnus Dei is a silver banner with red cross (as a direct allusion and symbol of the name of the County). On a heraldic chief of red is a gold castle with towers, with masonry joints in black, and with the windows and doors in red (in recognition of the fortress that was constructed in the military garrison of St. Augustine which is a part of the County).
An overall border is composed of eight parts; alternating, a red quadrilateral, with a gold castle and quadrilateral of silver with a purple lion rampant (that is to say, alternating the simplified Arms of Castile and Leon).
Given for a crest is a mural crown of a province. This is a circle of gold walls with in reality twelve gold towers with all the masonry joints in black. Only seven of the towers are visible in the drawing.
Said Arms as they are described and drawn may be used, having them engraved, sculpted and painted in the customary places, standing protected by this certification of Blazons, ratified, legalized and legitimized in the use of the same for Saint Johns County, in Florida (United States).