Same thing with the "too bright". Too bright based on what? Too bright for whom? Doesn't matter. Just follow these opin- I mean rules.
'Mathematical' alignment would mean centering on the basis of width or height, whereaas 'optical' aligment would adjust for the 'visual weight' of the item, which often 'feels' different.
For example, a 100px wide container with a 50px wide image within it, may not be appropriate to sit at the 25px->75px range. It may need to be slightly adjusted depending on the visual mass of the images.
Honestly these are excellent rules for design, and it's a bit funny in fact to see all the comments from people unable to process ostensible ambiguity in the rules, as though they are looking for a 'precision algorithm'.
I would say to anyone having difficulty processing why these are helpful rules, that it would be worth looking into design principless overall, it'll be a fun experience.
The documentary 'Helvetica' is pretty fun.
But apart from that, it is good advice, and I think the post gives a few more good rules of thumb too.