Standard 35mm full frame size was 24 x 36, which gives you the normal 3:2 format. If the 5:4 ish ones are a larger area than the normal 36x24mm slides, they may be transparencies from 127 film, which allowed images up to 40x40mm to be fitted in 2" x 2" slide mounts (the same size as 35mm film slides). These larger slides were known as "superslides" (see https://en.wikipedia.org/wiki/127_film#Superslide).

If the film area is the same size as the normal 35mm 3:2 images, but just with a black border, there are two possibilities: They may have been copied from 8 x 10 (or 20 x 16 (see note 1)) inch prints, or (if home mounted, or commercially processed with a clip shut plastic mount that can be opened and resealed) may have been masked down to that aspect ratio for artistic reasons.

Back in the film days, I used to do that with some of my slides - you could buy special slide mounts with a metal mask built in, or make your own mask from tinfoil (or possibly paper or card, but tinfoil was (a) opaque, and (b) non flammable - potentially important given that projector lamps can run pretty hot...).

You could also get APS slide film for the short lived APS cameras - these allowed a choice of the different APS aspect ratios, but - although they used the same 2" x 2" mounts, the film area was significantly smaller than 35mm.

note 1: 20 x 16 was a common size for mounted prints for photo society competitions (in the UK, at any rate - though that's probably shifted to the metric near equivalent (40 x 50 cm?) these days.)