I'm as surprised as you are. It's nice to not smack my head on doors or low-hanging ceiling attachments anymore
It's definitely not because of any bone changes. Starting HRT in youth can have some effects on bone shape, particularly in the pelvis/hips, but not at my age. Although, it's a well known side effect that feminising HRT can result in decrease in bone density, so we are often directed to increase calcium uptake etc.
For me it's like 3-4 cm more shortening than anyone else I've ever heard of, but I have quite a few close friends who've also lost a few inches in height. Trans healthcare is changing so rapidly that there really isn't much in the way of strong literature explaining long-term side effects of modern HRT treatments - they are just too new for a solid breadth of literature to have been developed yet. I'm on a very strong anti-androgen (which isn't even available to be prescribed in the US yet, for example) and the effects I've experienced have been beyond the expected changes across the board. I had D-cups in 3 months, as well, which is just about unheard of in literature/previous HRT, but my experiences and those of my friends/peers is suggesting that modern treatments may just be more effective than those that the literature actually covers.