I think it is, like one of the points you made, the idea that masculine women are to a degree acceptable in society but feminine men not so much. I almost think it's like there's a lot of women out there who are comfortable enough just dressing more masculine and acting a little more masculine but without having to go any further than that. So what I feel can happen is that some people like that will identify as transgender but not feel the need to actually go the route of going on hormones or getting surgery, whereas some just don't need to go that route and can stay happy being a masculine female without needing to change their name for example.
Before I continue I am more than well aware that there's more to transitioning than clothing, but this is just example based.
The "unisex" culture is more that male clothing is now considered for either gender, but there's no real female unisex thing happening. Female tees are female tees, there's no unisex about that. And female clothing is specific to a women's shape, but women can wear men's clothing, and they can even make male clothing look feminine. A man's shape is just more straight so if you have curves, you just might need a bigger size but it can work. Yet feminine clothing doesn't work so well without the body shape.
Also, with equality efforts and so on, more masculine based traits are actually a good thing for women in the work place and so on. For example, greater confidence, drive and being competitive. Whereas more feminine based traits of being soft and shy aren't as great qualities in many work places. So for women, it can be acceptable, and sometime encouraged in certain areas to be masculine.
So women have the option to be either feminine, masculine or combination of the two and that's all well and good to a degree. But there's no real alternative for men. A man can't really go unnoticed or be quite so accepted if they wear feminine clothing. That stands out. A feminine man stands out; a masculine woman...meh! And to really look right in female clothing, you do need to try and fill out certain areas. So for that people feel the need to either fake body parts or get on hormones and get surgery.
Basically the way I see it is that I personally believe that for FTM we only see the tip of the iceberg, as in the ones who more need medical intervention or at least a male name. And perhaps to a degree if having masculine traits wasn't as acceptable or even encouraged for women, we'd maybe actually have more people identifying as FTM so they could act and wear the clothes they want to.
I'm almost scared to post this for fear of looking like I'm making out that some MTFs aren't true MTFs or something. But I don't mean that at all. I mean that I feel societal influence can have an influence just based on what is acceptable and if not, what we have to potentially change to be acceptable as who we are inside. And I'm also not trying to say it's easier to be FTM because I actually find that the, "Why not just stay a masculine female because that's acceptable?" argument is common. I actually find some people suggesting they better understand MTF over FTM given the culture we have today.