It may be because you become (over) conscious of the actual act of typing. You start to think about the movement you make with your fingers, the position of your arms, posture etc. and this can be distracting or causing you to be nervous so you start to make more mistakes. I found that not concentrating on the actual typing but just trying to read the text you type as you would read a normal text helps me to relax and, therefore, not make so many mistakes.

Might also be that you try to process the information in the quote more carefully since you're interested, which might also slow you down. I must say I rarely encounter such quotes, and if I do, I tend to skip them because I'm not really looking for advice anymore, and they just bore me. The ones I do remember though often had some numbers in them, or capitalized abbrevations like WPM, which are awkward to type if you're not using the capslock key.

Anyway, I wouldn't let it bother me too much.

They are boring, aren't they..., no offense to anyone. I think I'll just skip them like you - I've already typed them a billion times (or so it seems :)...!