A simple test of the rules StackRanker3000 linkes to is whether you'd hold a PhD degree from those establishments in high regard independent of the field. If so, that's a universitet. If you'd only highly respect PhD degrees awarded in a few areas, like STEM fields when it comes to CalTech or MIT, that's a högskola, similar to an institute (possibly polytechnic if it is strong in more than one field).

And as was written above, some establishments, as they grew from institutes to universities, decided to keep the old name, MIT and CalTech included.