I realized that butter, when kept cool after buying, can be easily cut and spread when taken out of the fridge. If it gets warm once, however, it will be less soft the next time I take it out of the fridge. If it reaches room temperature once, it cannot even be cut when cooled down again to fridge temperature.
What is causing this phenomenon?
(I refer to real butter from cow milk, not what might be sold as butter in some countries)