Enchantments come in groups. Rerolls are following some rules, e.g. it’s only possible to have one enchant of a specific group. Some specific enchantments in a group have a higher probability to reroll as others. So quite often the reroll of the main group enchant will result in “indestructible” or “increased durability” due to their higher probability.
Your CSB drops are quite likely, but it all depends on which boss we are talking about. Darak pestilence loves to drop the CSB. While the cellarspiderboss in Marin Woods seems to really only drop the CSB as golden and that he does every second time. So it’s easy to accumulate a truckload of CSBs, if you would intentionally go after them.
Assuming that you are getting those indeed from mixed pestilence bosses, I’d speculate you are probably just a bit outside of the normal random distribution. Farm a little longer and things will most likely even out. I am not counting CSBs anymore, due to having run the cellarspiderguy too often 