Answering my own post here, in case anyone gets the same problem:

The reason for this behavior was because of my homestead-repository (the one that contains Vagrantfile and Homestead.yaml) was inside Dropbox, and I was using that same cloned repo on several computers. Every time "vagrant up" was called, vagrant created a hidden .vagrant-folder that contained meta-info on the newly created vm (among other things, an id). When I switched to a different computer and did "vagrant up", vagrant looked for the vm with that last id. Since that vm obviously was on a different machine altogether, vagrant created a new vm. Switch computer once again, and the same thing happens, and so on, and so forth...

The solution: Either move the homestead-repo outside of Dropbox and map virtual folders to the folders you want inside dropbox (what I did) or clone the homestead repo once for each computer you want to use it with (e.g. homestead imac, homestead pc and homestead work).