The wording is terrible. Yes.
But it seems clear that the intent is that each time you buy the Attribute Increase accessory it is taken for either Agility, Armor, or Strength. When you buy one rank of Attribute Increase it only increase either Agility, Armor, or Strength. That one rank of Attribute Increase does not increase all three attributes at once. The Attribute Increase accessory itself can only add either Agility, Armor, or Strength.
Each cyberlimb can have multiple cyberlimb accessories. The attribute increase accessory can be taken for each of the three attributes, independent of each other.
The only limiting factor here is capacity (and resources).
You can install an obvious cyberarm with for example 7 ranks of the Increase Agility accessory (for a total Agility of 9) and 8 ranks of the Increase Strength accessory (for a total Strength of 10). If you like.
....even if you only have a natural Agility and Strength of say, 3.
Now, since the maximum augmented increase of 4 is in place here you would only be allowed to utilize an Agility rating of 3+4=7 and Strength rating of 3+4=7 (which mean the last 2 ranks of your Increased Agility accessory and the last 3 ranks of Increased Strength accessory would currently be 'wasted').
Having said that, if you later, during game play, raise your natural Agility from 3 to 5 and your natural Strength from 3 to 6 you would actually be allowed to utilize the full scope of your obvious cyberarm (since Agility 5+4=9 and Strength 6+4=10).