The codes used for aviation purposes ie ICAO aren't necessarily the same as the codes used for travel ie IATA, or commonly used abbreviations for a place.
ICAO uses a 4 letter code, IATA uses 3 letters. The first ICAO code letter is specific to a geographic region eg 'Y' for Australia, 'E' for parts of Europe, 'K' for the US. The remaining 3 letters must then be unique to that location. Bear in mind that there can be a number of airports serving a particular city, not all concerned with airline travel, but all must have a unique identifier - hence the large variety of ICAO codes.
IATA, on the other hand, is only concerned with airports used for airline travel. They tend/try to use a contraction of the city/town served by the airport (apart from historical anomalies such as Orlando International's 'MCO' taken from its original name of McCoy Field). IATA also has to allow for different terminals within a particular airport hence 'LHR' is Heathrow but 'LH1' is Heathrow's Terminal 1.
The US, by & large, manages to keep ICAO & IATA similar if they can. This gives the ICAO code of KMCO for Orlando Int'l, and the IATA code of MCO. It's made more difficult when all the navaids in an area must also be named. Can't use the same name for everything so some alterations/renaming is done to ensure they can be distinguished from each other.
The US does a better job of it than say, the UK. the UK makes no attempt to keep the two codes similar. For example Sumburgh is ICAO 'EGPB', IATA is 'SUM'.
Immigration has more than just aviation ports to name & code for. They have to include sea ports, land border crossings etc so they may desire or need additional or different codes again.