Okay, so, I want to start out by saying that I've used Japanese planes a lot.
The first hand plane that I learned how to use, even, was a little inexpensive Japanese block plane.
So I've used Japanese planes for many years now, but I have always used my western planes more, and so I've developed more skill with them than I have Japanese planes I suppose.
Now, with Western planes, I can plane with great confidence and shave down to a specific depth quite straight and consistently, IE, I get a consistently thick shaving across the entire length of the board, and don't have to think about this.
However, with Japanese planes, I have to be very careful, because if I just take shavings from one end of the board to the other, I consistently wind up with a taper on the piece (meaning that I am taking more material off of one end of the board than the other). This is not so noticeable with just a few shavings, but if I take a bunch of shavings this way, whatever I am doing adds up to create a taper. If you were to look at the shavings individually, they're not apparently thicker on one end than the other, at least not to the naked eye.
I think it is generally the far end, where I start the shaving, that remains thicker / has less material removed. I haven't been able to discern what it is I am doing that is making this happen, though. The blade does engage and start cutting at the very end, as far as I am aware, and I'm not generally making a "dip" off the end of the board either, as far as I'm aware. Maybe I'm putting more downwards force at the end when it's closer to my body, and this is somehow engaging the iron ever so slightly deeper? I don't know.
I'm curious if anyone else has found this to occur with Japanese planes, and, more importantly, has any idea why it would.
I'd like to add that this has occurred with every Japanese plane that I've owned, which has been quite a few. So I don't think it's just a matter of the Dai being off on one particular plane.
I have a feeling it's to do with the mechanics of starting and ending the cut on the pull stroke, or where the front of the plane is longer than the end of it, and there's just some ever so slight thing that I'm doing wrong with where I place the pressure. Typically, I do as one would with a western plane, placing the pressure on the front when starting, and the end when ending, the cut.