Think of all the things we do to our fragile hair. Our hair is different, our growth rate is NOT dfferent. the average human being-- regardless of race-- will get that .5 inches. On average, how many of us here get that without growth aids? Let's do a poll and we'll see the bell curve at 0.5.
The reason our hair is shorter is because our hair is breaking off just as fast, if not faster than it's growing. The same just is not true about other women, and in most cases it has been that way since we were very young kids. Most of these other women aren't 25 and just now learning/growing after being EL all their lives. Many of us are starting out that way because our parents did not have the knowledge. I watched my niece's hair progress from two pigtails at an average SL length at 3, to when they started complaining that her hair was "unmanageable" and "unruly" as her texture changed with age, and it started to break off to where it will likely be until someone shows her better than kiddie perms and pressing combs and rattail combs and hair grease. Or think of how many of us had longer hair as a young child when our moms just used to put us in 2 braided pigtails and called it a day, vs. when you got to high school and started thinking you were cute & cool and doing all of this other stuff to your hair like relaxing?
Once we get the knowledge of how to properly care for it, we start to fare better as evidenced here, but even then many of us are still having breakage just because our hair is super fragile.
And look around-- BSL, MBL, WL is rare on other chicks as well. I've been paying more attention to everyone's hair length lately, and on other women... they are usually between SL and APL. It's hard for us to notice that just because it's more than what we see in our own communities, but pay extra attention when you are at work or in other neighborhoods. I see about 75% SL and APL.
6 inches a year is the average, regardless. Retention is the key. And yes, retention is much more difficult for us. Yet almost everyone I know with dredlocks has hair that is past APL, BSL... WL & beyond if it could be straightened. Imagine that.