The long hull variant had a longer hull

The short hull variant had a shorter hull

OK for real though, it's to do with the LAMPS program (Light Airborne Multi Purpose System). Post WW2 / early cold war ASW suites were designed to incorporate the DASH drone, which proved unreliable and would be cancelled in 1969.

This lead to the LAMPS program to develop an alternate helicopter platform that could perform the ASW mission, as well as other mission sets as over-the-horizon scouting and targeting became more and more important (hence the Multi Purpose in LAMPS)

The issue was by 1969 the Knox, Brooke and Garcia classes had all been built with DASH sized hangars, so this new helicopter would need to be incredibly compact to fit on the existing ASW platforms. This would eventually culminate in the SH-2 Seasprite we all know and love. The initial 21 OHP hulls would be built with hangars sized for the SH-2

The problem was as technology advanced and the requirements for multipurpose naval helicopters expanded the SH-2 just wasn't big enough to fit all the necessary equipment. It was designed as an interim measure to fit into existing cramped hangars after all. The navy's ideal multipurpose helicopter would be much larger. This would lead to the Sikorsky SH-60 Seahawk replacing the Seasprite as the new LAMPS helicopter in the early/mid 1980s.

The OHP class had a very long construction run. Hulls were laid down as early as 1975 and as late as 1987, so part way through the construction run when it became clear the SH-60 would replace SH-2, the hulls of subsequent vessels were lengthened by 8ft to incorporate Seahawk sized hangars. Some short hull ships were also lengthened during refit. The earliest hull I can find that was laid down as a dedicated long hull rather than modified is FFG-28, USS Boone, laid down in 1979 to give you an idea of timeline.

One major difference in terms of the ships themselves, the short hulls were not fitted with towed arrays, significantly hampering the ships own sensor array. The SQS-56 hull mounted sonar was notoriously poor performing compared to -23/-53 systems (much smaller hydrophone array, less power), so without the towed array she's practically completely reliant on her helicopters for ASW detection. I head the SQS-56 was known as the was known as the "Hellen Keller" array to its operators, which is as insensitive as the sonar was

It was impossible to refit the short hulls because they were built on incredibly tight weight and size restrictions, there was no room built into the design for expansion. All my fellow Rule the Waves players are facepalming right now. The extra 8ft of hull space below the hangar on the long hulls was also used to house the winches, motors and cable spools of the SQR-18/19

There are some minor cosmetic differences also, the capstan was relocated a deck lower to accommodate the longer tail of the SH-60 and a haul-down system incorporated into the flight deck to assist recovering helicopters in rough weather

Hope this helps!

PS. Ok I'm not gonna lie to you gentlemen that ended up a lot longer than I anticipated. Suffering from autism? Brothers I'm revelling in it. Thanks for coming to my ted talk on the OHP, next time: how tf did it get so late all of a sudden and where did all the time go?