In which book is Redliner described?
So yeah a human can be 1 agi and strength, but buying enhancements can have the limbs at 6(9) with the +3 customisation
You can
customize it up to metatype maxium (in this case 5 points up to 6) and you can
enhance them to +3, but yes - a human with agility 1 can get a cyberarm with agility 6(9).
Natural ratings of the body can be increased post chargen by spending Karma up to the metatype maximum. Beyond that you can augment your body. Attributes of the natural body normally have an augmented maximum of +4 (but you can break it with for example wireless enabled wired reflexes + wireless enabled reaction enhancers).
Cyberlimbs can be customized up to natural metatype maximum at installment no matter the current rating of the natural body. You can not customize the cyberlimb after installment. Cyberlimbs have attributes of their own, independent of the rest of your body. Beyond customization you can enhance cyberlimbs. Attributes of cyberlimbs have an enhancement maximum of +3.
Having said that, rules as they are written are stupid. They discourage already physically strong characters (like a troll samurai) to get chromed limbs while they encourage physically weak characters (like a decker) to get a cyberarm.
IMO in a cyberpunk universe you would much more often see a physically strong samurai with chromed arms than a decker. Rules should be changed to reflect this. Also a body with strength 1 that have cyberarms with enhanced strength of 9 should probably risk ripping his own arms out of their sockets ;-)
I think it would make more sense if they capitalized on the following fluff:
You can have your cyberlimb tailored and customized to your frame and musculature, to make customization limited by current natural attributes in some way.