Why are 318's low HP and torque?

KB has an online catalog:

[ame]http://www.kb-silvolite.com/assets/automotive_catalog.pdf[/ame]

I see 2 318 stock stroke pistons listed:

KB167 KB167 KTM 2-5/64, 1-3/16 FT 5cc 2V 10.4 10.1 9.6 9.2 507/132 .275 .280-2.160" .280-2.160"

OR

KB399 KB399 KTM 2-5/64, 1-3/16 .140” SD -6cc 2V 12.2 11.7 11.0 10.4 541/132 .275 .286-2.200" .286-2.200"