I have been looking at those jagged textures, too. There are macro pixels in these textures that, in my case, playing at 1024 x 768 screen resolution, are blocks of 8 x 8 = 64 screen pixels. According to Asaki in a previous post,
"It has nothing to do with AF or AA, it has to do with the low resolution textures being upscaled before bilinear filtering is applied."
I think that means that the textures are originated as much smaller images in terms of screen pixel dimensions. They must be up-scaled (resized) to fit the screen dimensions wanted. That is the factor of 8. It is a question of whether they are up-scaled before or after the filtering is applied. Clearly, if you do the filtering after the texture is upscaled (by a factor of 8), it will not affect the blocky appearance so much.
Another factor is that if you have higher resolution textures to begin with, then there will not be such extensive blocking.
Now, as to why that is actually happening, I haven't the slightest idea.