Hello all,
I am a tool and die maker by trade. The company I work for utilizes injection molders and extruders as a part of our production line, so maybe I can shed some light on a couple things.
The texture on the sloped bricks is most likely produced via plunge EDM (Electrical discharge machining). I am pretty sure that all of the Lego logos and part numbers are applied to the molds via some kind of EDM machining. Depending on where the mold is made, this "pattern" can vary greatly depending on who makes the mold. If the molds are being made all over the world, there could definitely be some kind of disparity between the patterning in the individual molds themselves. The variables would be the machine itself, the tool material used to etch the pattern and even the parameters used to "burn" the pattern into the mold. Different rates of electrical discharge will produce different results.
As for the molds wearing out, this is another feasible conclusion. Plastic is actually extremely abrasive when being forced with several thousand PSI of force into a small cavity (like a brick mold). Molds wear a lot quicker then one would generally think when certain plastics are being used. A couple thousandths of an inch of wear will mean the difference between the blocks fitting and not fitting. For those of you that do not have a good idea what a thousandth of an inch is, 3 thousandths is equal to roughly the width of an average human hair. So rip a hair out of your head and then cut it lengthwise into 3 equal pieces...that a thousandth (.001") X-D
Things I have noticed on bricks recently:
Large sprue marks on the bricks...they use to be nearly undetectable.
The LEGO logo's are a lot less neat.
Gaps between butt-joints with bricks. There never use to be a gap between bricks when attached to each other.
If you have any more questions about machining or injection molding, please let me know...if I do not have the answer, I can get the answer through my contacts at work.