You only get benefits from your strength or defense at certain numbers.
See
http://idb.finalhit.org/?searchtype=pag ... es&a_id=80 for numbers.
So if you have a 8-icon weapon and strength 7, you have 0.5 multiplier and you'll do 6*0.5 3 HP of damage.
If you have strength 8, you have the 0.75 multiplier and do 6 damage.
If you get up to strength 12, you haven't reached the next boost yet, so you still do only 6 damage.
But as soon as you get to strength 13, you reached the 1.0 multiplier, and you do 8 damage with that weapon.
... and so on ...
Defense works about the same way - you end up being able to block x HP worth of whatever-element damage.
Hope that helps