Well one reason is that the viewing angle looking down in the shower is significantly different from looking into a flat mirror. We also don't see our own face while looking down, so our mind can't automatically trick us into thinking 'oh, that is just me, the same me I have always been.'
Another reason is that the image we see in the mirror is the one we have seen every day of our adult lives. It takes a while for the changes to register in our minds. Until our minds catch up we see a residual image of ourselves. People who don't see us very often will probably start noticing differences after 3 - 6 months.
I have been on HRT for 9 months. Most of the time when I look in the mirror I have a hard time noticing any differences, I don't think I have changed much. However, when I look at a recent photo and compare it to one taken before I started HRT I can see significant changes.
My oldest daughter came to visit for Christmas. We had not seen each other since April. Until two days ago she did not know I was transitioning. Once I told her I asked if she had noticed anything. She said that I definitely looked different, but she didn't know exactly what the differences were.
